shutdown: propagate mount() failures from child to parent

Let's propagate the actual error code up, as we usual do.

Inspired by: #25168

src/basic/process-util.h

@@ -68,6 +68,8 @@ int wait_for_terminate_with_timeout(pid_t pid, usec_t timeout);
 void sigkill_wait(pid_t pid);
 void sigkill_waitp(pid_t *pid);
 void sigterm_wait(pid_t pid);
+void sigkill_nowait(pid_t pid);
+void sigkill_nowaitp(pid_t *pid);
 
 int kill_and_sigcont(pid_t pid, int sig);