explicitly test set[ug]id() return values

Legacy !_POSIX_SAVED_IDS path only; coverity report via Ed Maste
ok dtucker@

uidswap.c

@@ -162,8 +162,10 @@ restore_uid(void)
 	 * Propagate the real uid (usually more privileged) to effective uid
 	 * as well.
 	 */
-	setuid(getuid());
-	setgid(getgid());
+	if (setuid(getuid()) == -1)
+		fatal("%s: setuid failed: %s", __func__, strerror(errno));
+	if (setgid(getgid()) == -1)
+		fatal("%s: setgid failed: %s", __func__, strerror(errno));
 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
 
 	if (setgroups(saved_egroupslen, saved_egroups) == -1)