daemon/shell: refactor redirection of 0/1/2 from /dev/null

Both daemon.c and shell.c contain logic to open FDs 0/1/2 from
/dev/null if they are not already open.  Move the function in daemon.c
to setup.c and use it in shell.c, too.

While there, remove a 'not' that inverted the meaning of the comment.
The point is indeed to *avoid* messing up.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -872,3 +872,15 @@ const char *resolve_gitdir(const char *suspect)
 		return suspect;
 	return read_gitfile(suspect);
 }
+
+/* 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");
+	if (fd > 2)
+		close(fd);
+}