netcat: Add UNIX domain socket support (-U)

Other implementations of netcat support UNIX domain sockets via -U, so
this change adds it to toybox too.

Example usage:
Create ./uds as a UNIX domain socket and listen on it, forwarding
messages to bash:
> nc -U -s ./uds -L /bin/bash

Connect to ./uds as a UNIX domain socket
> nc -U ./uds
1 file changed