login: remember that fds received from PID1 need to be removed eventually

Remember to set sd->pushed_fd when we receive an fd from PID1 on startup,
the same as we set it when we send an fd to PID1.

src/login/logind-session-device.c

@@ -543,5 +543,6 @@ void session_device_attach_fd(SessionDevice *sd, int fd, bool active) {
         assert(!sd->active);
 
         sd->fd = fd;
+        sd->pushed_fd = true;
         sd->active = active;
 }