windows: msvc: Define STDIN/OUT/ERR_FILENO.

Because they are not available we've used _fileno(stdin) in some places, but
that doesn't reliably work, because stdin might be closed. This is the
prerequisite of the subsequent commit, fixing a failure introduced in
76e38b3.

Author: Andres Freund <andres@anarazel.de>
Reported-By: Sandeep Thakkar <sandeep.thakkar@enterprisedb.com>
Message-Id: 20220520164558.ozb7lm6unakqzezi@alap3.anarazel.de (on pgsql-packagers)
Backpatch: 15-, where 76e38b3 came in

Author: anarazel

src/include/port/win32_msvc/unistd.h

@@ -1 +1,9 @@
 /* src/include/port/win32_msvc/unistd.h */
+
+/*
+ * MSVC does not define these, nor does _fileno(stdin) etc reliably work
+ * (returns -1 if stdin/out/err are closed).
+ */
+#define STDIN_FILENO	0
+#define STDOUT_FILENO	1
+#define STDERR_FILENO	2