Enable Unix-domain sockets support on Windows

 Enable Unix-domain sockets support on Windows
 
 As of Windows 10 version 1803, Unix-domain sockets are supported on
 Windows.  But it's not automatically detected by configure because it
 looks for struct sockaddr_un and Windows doesn't define that.  So we
 just make our own definition on Windows and override the configure
 result.
 
 Set DEFAULT_PGSOCKET_DIR to empty on Windows so by default no
 Unix-domain socket is used, because there is no good standard
 location.
 
 In pg_upgrade, we have to do some extra tweaking to preserve the
 existing behavior of not using Unix-domain sockets on Windows.  Adding
 support would be desirable, but it needs further work, in particular a
 way to select whether to use Unix-domain sockets from the command-line
 or with a run-time test.
 
 The pg_upgrade test script needs a fix.  The previous code passed
 "localhost" to postgres -k, which only happened to work because
 Windows used to ignore the -k argument value altogether.  We instead
 need to pass an empty string to get the desired effect.
 
 The test suites will continue to not use Unix-domain sockets on
 Windows.  This requires a small tweak in pg_regress.c.  The TAP tests
 don't need to be changed because they decide by the operating system
 rather than HAVE_UNIX_SOCKETS.
 
 Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
 Discussion: https://www.postgresql.org/message-id/flat/54bde68c-d134-4eb8-5bd3-8af33b72a010@2ndquadrant.com