DEV: tcploop: add minimal support for unix sockets

Since the tool permits to pass an FD bound for listening, it's convenient
to test haproxy's "bind fd@". Let's add support for UNIX sockets the same
way. -U needs to be passed to change the default address family, and the
address must contain a "/".

E.g.
  $ dev/tcploop/tcploop -U /tmp/ux L Xi ./haproxy -f fd1.cfg
1 file changed