Add examples for IO::Socket::UNIX.

In the SYNOPSIS.

See
http://www.nntp.perl.org/group/perl.perl5.porters/2014/01/msg211274.html:

<QUOTE>
From: Shlomi Fish
Subject: Patch for perldoc IO::Socket::UNIX

I noticed http://perldoc.perl.org/IO/Socket/UNIX.html does not contain any
examples (including not in the synopsis). If acceptable, I can a prepare a
patch that will include a simple server and a client for UNIX domain sockets,
see: http://www.mail-archive.com/beginners%40perl.org/msg116942.html
</QUOTE>