Lower privilege level of programs calling regression_main

On Windows this mean that the regression tests can now safely and
successfully run as Administrator, which is useful in situations like
Appveyor. Elsewhere it's a no-op.

Backpatch to 9.5 - this is harder in earlier branches and not worth the
trouble.

Discussion: https://postgr.es/m/650b0c29-9578-8571-b1d2-550d7f89f307@2ndQuadrant.com

src/test/regress/pg_regress.c

@@ -2081,6 +2081,8 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 	progname = get_progname(argv[0]);
 	set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("pg_regress"));
 
+	get_restricted_token(progname);
+
 	atexit(stop_postmaster);
 
 #ifndef HAVE_UNIX_SOCKETS