machine_kill(): Don't kill the unit when killing the leader

If "machinectl poweroff" or "machinectl reboot" is used on a
systemd-nspawn container started with --keep-unit and --register, it
should *only* send the appropriate signal to the leader PID (i.e. the
container's systemd process). It shouldn't fall through to
manager_kill_unit() to also send the signal to the unit. The latter
ends up killing systemd-nspawn, which takes down the container
prematurely.

src/machine/machine.c

@@ -493,6 +493,8 @@ int machine_kill(Machine *m, KillWho who, int signo) {
 
                 if (kill(m->leader, signo) < 0)
                         return -errno;
+
+                return 0;
         }
 
         /* Otherwise make PID 1 do it for us, for the entire cgroup */