On shutdown, return ISC_R_SHUTTINGDOWN from isc_taskmgr_excltask()

On shutdown, return ISC_R_SHUTTINGDOWN from isc_taskmgr_excltask()

The isc_taskmgr_excltask() would return ISC_R_NOTFOUND either when the
exclusive task was not set (yet) or when the taskmgr is shutting down
and the exclusive task has been already cleared.

Distinguish between the two states and return ISC_R_SHUTTINGDOWN when
the taskmgr is being shut down instead of ISC_R_NOTFOUND.