udevd: fix crash when workers time out after exit is signal caught

If udevd receives an exit signal, it releases its reference on the udev
monitor in manager_exit(). If at this time a worker is hanging, and if
the event timeout for this worker expires before udevd exits, udevd
crashes in on_sigchld()->udev_monitor_send_device(), because the monitor
has already been freed.

Fix this by releasing the main process's monitor ref later, in
manager_free().