shutdown: unify shutdown.c's and async.c's sync() helper process

The helper processes are pretty much the same now, let's unify them
hence.

src/basic/async.h

@@ -22,5 +22,5 @@
 
 int asynchronous_job(void* (*func)(void *p), void *arg);
 
-int asynchronous_sync(void);
+int asynchronous_sync(pid_t *ret_pid);
 int asynchronous_close(int fd);