This wraps msghdr on Unix and WSAMSG on Windows to be used in
send/recvmsg calls.

Wrapper around sendmsg(2).

Instead we'll create a separate MsgHdrMut for recvmsg, similar to
IoSlice and IoSliceMut.

To be used in recvmsg(2) calls.

pointer::cast_mut was stablised in 1.65, so it's too new.

Smoke tested on esp32c3 dev board.  I've also tested a similar patch
backported to v0.4.9 with much greater functionality including tokio +
mio with other patches I've been working on and it's fully working.

Closes rust-lang#379

This was missed in rust-lang#452 because I wasn't testing with feature="all"
enabled for my small socket2 test.  For the full tokio integration I was
using v0.4.x which didn't need this fix.

Properly closes rust-lang#379.