core: all unit types that watch control PIDs should use the same logic

When coldplugging the unit state, make sure to follow the same basic
logic for all unit types: always verify whether the control PID is still
a waitable process before proceeding.

src/core/socket.c

@@ -1455,7 +1455,9 @@ static int socket_coldplug(Unit *u) {
         if (s->deserialized_state == s->state)
                 return 0;
 
-        if (IN_SET(s->deserialized_state,
+        if (s->control_pid > 0 &&
+            pid_is_unwaited(s->control_pid) &&
+            IN_SET(s->deserialized_state,
                    SOCKET_START_PRE,
                    SOCKET_START_CHOWN,
                    SOCKET_START_POST,
@@ -1466,9 +1468,6 @@ static int socket_coldplug(Unit *u) {
                    SOCKET_FINAL_SIGTERM,
                    SOCKET_FINAL_SIGKILL)) {
 
-                if (s->control_pid <= 0)
-                        return -EBADMSG;
-
                 r = unit_watch_pid(UNIT(s), s->control_pid);
                 if (r < 0)
                         return r;