Make sure pg_rewind can't run as root

Previously a warning was printed, but the tool actually kept running
even when running as root. This is something we definitely want to
prevent, but since this means a behavior change, not backpatching.

Author: Michael Paquier

src/bin/pg_rewind/pg_rewind.c

@@ -208,6 +208,7 @@ main(int argc, char **argv)
 		fprintf(stderr, _("cannot be executed by \"root\"\n"));
 		fprintf(stderr, _("You must run %s as the PostgreSQL superuser.\n"),
 				progname);
+		exit(1);
 	}
 #endif