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

src/include/pg_config_manual.h

@@ -135,13 +135,6 @@
 #define EXEC_BACKEND
 #endif
 
-/*
- * Disable UNIX sockets for certain operating systems.
- */
-#if defined(WIN32)
-#undef HAVE_UNIX_SOCKETS
-#endif
-
 /*
  * USE_POSIX_FADVISE controls whether Postgres will attempt to use the
  * posix_fadvise() kernel call.  Usually the automatic configure tests are
@@ -202,8 +195,16 @@
  * server will not create an AF_UNIX socket unless the run-time configuration
  * is changed, a client will connect via TCP/IP by default and will only use
  * an AF_UNIX socket if one is explicitly specified.
+ *
+ * This is done by default on Windows because there is no good standard
+ * location for AF_UNIX sockets and many installations on Windows don't
+ * support them yet.
  */
+#ifndef WIN32
 #define DEFAULT_PGSOCKET_DIR  "/tmp"
+#else
+#define DEFAULT_PGSOCKET_DIR ""
+#endif
 
 /*
  * This is the default event source for Windows event log.