From Jason Tishler <jt@dothill.com>

* doc/FAQ_MSWIN: Update to be consistent with software -- mainly change
comment from lack of Cygwin UNIX domain socket support and to list of
current Cygwin UNIX domain socket issues.
* src/include/config.h.in: Enable UNIX domain sockets for Cygwin.
* src/include/port/win.h: Disable UNIX domain sockets for Cygwin b20.1.
* src/test/regress/pg_regress.sh: Use UNIX domain sockets for Cygwin
instead of TCP/IP.