fix a possible use-after-detach bug in rndc

we need to detach the rndc task when calling isc_app_shutdown(), otherwise it may be detached before the last reference to it. while this does not prevent rndc from working, it causes a spurious core dump on shutdown.

NOTE: The above rationale may not be entirely accurate. See a discussion elsewhere for more details.

Closes #3520 (closed)