Adding checks for fd omission

Adding function check_fds to new file fd.c. The function check_fds
should be called in every setuid/setgid program.

Co-developed-by: Alejandro Colomar <alx@kernel.org>

src/chfn.c

@@ -620,10 +620,12 @@ int main (int argc, char **argv)
 	char                 *user;
 	const struct passwd  *pw;
 
+	sanitize_env ();
+	check_fds ();
+
 	log_set_progname(Prog);
 	log_set_logfd(stderr);
 
-	sanitize_env ();
 	(void) setlocale (LC_ALL, "");
 	(void) bindtextdomain (PACKAGE, LOCALEDIR);
 	(void) textdomain (PACKAGE);