setup: use xopen and xdup in sanitize_stdfds

Replace the catch-all error message with specific ones for opening and
duplicating by calling the wrappers xopen and xdup.  The code becomes
easier to follow when error handling is reduced to two letters.

Remove the unnecessary mode parameter while at it -- we expect /dev/null
to already exist.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -1423,11 +1423,9 @@ const char *resolve_gitdir_gently(const char *suspect, int *return_error_code)
 /* if any standard file descriptor is missing open it to /dev/null */
 void sanitize_stdfds(void)
 {
-	int fd = open("/dev/null", O_RDWR, 0);
-	while (fd != -1 && fd < 2)
-		fd = dup(fd);
-	if (fd == -1)
-		die_errno(_("open /dev/null or dup failed"));
+	int fd = xopen("/dev/null", O_RDWR);
+	while (fd < 2)
+		fd = xdup(fd);
 	if (fd > 2)
 		close(fd);
 }