Use _exit, not exit in forked process

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

tools/regression/sockets/accept_fd_leak/accept_fd_leak.c

@@ -170,7 +170,7 @@ main(void)
 				errx(-1, "connect: %s", strerror(errno));
 			close(s);
 		}
-		exit(0);
+		_exit(0);
 	}
 
 	/* Reset back to a blocking socket. */