This reverts commit 0dfe464.

Revert:

• detach-netns: simplify unshare helper #497

Due to a regression moby/buildkit#5825 (comment)

This might be a regression in v2.3.3

• test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)

=== FAIL: frontend/dockerfile TestIntegration (0.05s)
    run.go:315: copied local:/mainline.tar to local mirror localhost:44447/buildkit_test/mtd7m2emwrrj5deqz9jduk2hb:latest
time="2025-03-10T01:18:46Z" level=info msg="fetch failed after status: 404 Not Found" host="localhost:44447"
    run.go:315: copied docker.io/amd64/alpine:latest@sha256:25fad2a32ad1f6f510e528448ae1ec69a28ef81916a004d3629874104f8a7f70 to local mirror localhost:44447/library/alpine:latest
time="2025-03-10T01:18:47Z" level=info msg="fetch failed after status: 404 Not Found" host="localhost:44447"
    run.go:315: copied docker.io/amd64/busybox:stable-musl to local mirror localhost:44447/library/busybox:stable-musl
time="2025-03-10T01:18:48Z" level=info msg="fetch failed after status: 404 Not Found" host="localhost:44447"
    run.go:315: copied docker.io/amd64/busybox:latest@sha256:023917ec6a886d0e8e15f28fb543515a5fcd8d938edb091e8147db4efed388ee to local mirror localhost:44447/library/busybox:latest
    run.go:315: copied docker.io/amd64/debian:bullseye-20230109-slim@sha256:1acb06a0c31fb467eb8327ad361f1091ab265e0bf26d452dea45dcb0c0ea5e75 to local mirror localhost:44947/amd64/debian:bullseye-20230109-slim

• test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)

== FAIL: frontend/dockerfile TestIntegration (0.09s)
    run.go:315: copied docker.io/amd64/alpine:latest@sha256:25fad2a32ad1f6f510e528448ae1ec69a28ef81916a004d3629874104f8a7f70 to local mirror localhost:42351/library/alpine:latest
time="2025-03-10T01:18:45Z" level=info msg="fetch failed after status: 404 Not Found" host="localhost:42351"
    run.go:315: copied docker.io/amd64/busybox:stable-musl to local mirror localhost:42351/library/busybox:stable-musl
time="2025-03-10T01:18:46Z" level=info msg="fetch failed after status: 404 Not Found" host="localhost:42351"
    run.go:315: copied docker.io/amd64/busybox:latest@sha256:023917ec6a886d0e8e15f28fb543515a5fcd8d938edb091e8147db4efed388ee to local mirror localhost:42351/library/busybox:latest
    run.go:315: copied docker.io/amd64/debian:bullseye-20230109-slim@sha256:1acb06a0c31fb467eb8327ad361f1091ab265e0bf26d452dea45dcb0c0ea5e75 to local mirror localhost:35903/amd64/debian:bullseye-20230109-slim

• test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....

=== FAIL: frontend TestFrontendIntegration/TestRefStatFile/worker=oci-rootless-slirp4netns-detachnetns (15.12s)
    run.go:238: 
        	Error Trace:	/src/util/testutil/integration/run.go:238
        	Error:      	Received unexpected error:
        	            	failed dialing: /tmp/bktest_buildkitd3359171510/buildkitd.sock
        	            	github.com/moby/buildkit/util/testutil/integration.WaitSocket
        	            		/src/util/testutil/integration/util.go:138
        	            	github.com/moby/buildkit/util/testutil/workers.runBuildkitd
        	            		/src/util/testutil/workers/util.go:115
        	            	github.com/moby/buildkit/util/testutil/workers.(*OCI).New
        	            		/src/util/testutil/workers/oci.go:85
        	            	github.com/moby/buildkit/util/testutil/integration.newSandbox
        	            		/src/util/testutil/integration/sandbox.go:127
        	            	github.com/moby/buildkit/util/testutil/integration.Run.func2.1
        	            		/src/util/testutil/integration/run.go:237
        	            	testing.tRunner
        	            		/usr/local/go/src/testing/testing.go:1690
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1700
        	            	creating worker
        	            	github.com/moby/buildkit/util/testutil/integration.newSandbox
        	            		/src/util/testutil/integration/sandbox.go:129
        	            	github.com/moby/buildkit/util/testutil/integration.Run.func2.1
        	            		/src/util/testutil/integration/run.go:237
        	            	testing.tRunner
        	            		/usr/local/go/src/testing/testing.go:1690
        	            	runtime.goexit
        	            		/usr/local/go/src/runtime/asm_amd64.s:1700
        	Test:       	TestFrontendIntegration/TestRefStatFile/worker=oci-rootless-slirp4netns-detachnetns
2025/03/10 01:21:19 stdout: /usr/bin/sudo -u #1000 -i -- exec rootlesskit --net=slirp4netns --copy-up=/etc --disable-host-loopback --detach-netns buildkitd --oci-worker=true --containerd-worker=false --oci-worker-gc=false --oci-worker-labels=org.mobyproject.buildkit.worker.sandbox=true --config=/tmp/bktest_config3279013865/buildkitd.toml --root /tmp/bktest_buildkitd43008409 --addr unix:///tmp/bktest_buildkitd43008409/buildkitd.sock --debug
2025/03/10 01:21:19 stderr: /usr/bin/sudo -u #1000 -i -- exec rootlesskit --net=slirp4netns --copy-up=/etc --disable-host-loopback --detach-netns buildkitd --oci-worker=true --containerd-worker=false --oci-worker-gc=false --oci-worker-labels=org.mobyproject.buildkit.worker.sandbox=true --config=/tmp/bktest_config3279013865/buildkitd.toml --root /tmp/bktest_buildkitd43008409 --addr unix:///tmp/bktest_buildkitd43008409/buildkitd.sock --debug
2025/03/10 01:21:19 > StartCmd 2025-03-10 01:21:04.558913329 +0000 UTC m=+0.030075216 /usr/bin/sudo -u #1000 -i -- exec rootlesskit --net=slirp4netns --copy-up=/etc --disable-host-loopback --detach-netns buildkitd --oci-worker=true --containerd-worker=false --oci-worker-gc=false --oci-worker-labels=org.mobyproject.buildkit.worker.sandbox=true --config=/tmp/bktest_config3279013865/buildkitd.toml --root /tmp/bktest_buildkitd43008409 --addr unix:///tmp/bktest_buildkitd43008409/buildkitd.sock --debug
2025/03/10 01:21:19 [rootlesskit:child ] error: failed to create a detached netns on "/tmp/rootlesskit2584759687/netns": failed to Statfs "/proc/14943/task/15004/ns/net": no such file or directory
2025/03/10 01:21:19 > sending sigterm 2025-03-10 01:21:19.6604122 +0000 UTC m=+15.131574057
2025/03/10 01:21:19 > sending SIGTERM 2025-03-10 01:21:19.660419885 +0000 UTC m=+15.131581741
2025/03/10 01:21:19 > stopped 2025-03-10 01:21:19.661570195 +0000 UTC m=+15.132732052 signal: terminated -1
    --- FAIL: TestFrontendIntegration/TestRefStatFile/worker=oci-rootless-slirp4netns-detachnetns (15.12s)

Originally posted by @AkihiroSuda in moby/buildkit#5825 (comment)