Do port detection in cdc_rsync_server and cdc_fuse_fs instead of running netstat/ss

[ljusten]

Running netstat/ss via ssh to find available ports is slow and not very robust. It would be better to detect ports in C++ code. One problem right now is when cdc_rsync_server is started, the port must be known as port forwarding is set up at the same time.

Instead doing this:

• Run netstat locally and remotely to find available ports
• Run an ssh command that sets up port forwarding and executes cdc_rsync_server / cdc_fuse_fs
• Connect to port

Do this:

• Run an ssh command that executes cdc_rsync_server / cdc_fuse_fs
• In cdc_rsync_server / cdc_fuse_fs, detect available ports (in C++ code!), print out "...is listening on port X" and block
• In cdc_rsync / cdc_stream, read port, find available local port (in C++ code!), set up port forwarding and connect to port