config_intrhook: provide config_intrhook_drain

config_intrhook_drain will remove the hook from the list as
config_intrhook_disestablish does if the hook hasn't been called. If it has,
config_intrhook_drain will wait for the hook to be disestablished in the normal
course (or expedited, it's up to the driver to decide how and when
to call config_intrhook_disestablish).

This is intended for removable devices that use config_intrhook and might be
attached early in boot, but that may be removed before the kernel can call the
config_intrhook or before it ends. To prevent all races, the detach routine will
need to call config_intrhook_train.

Sponsored by: Netflix, Inc
Reviewed by: jhb, mav, gde (in D29006 for man page)
Differential Revision: https://reviews.freebsd.org/D29005