Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r254813,259587: Rename the kld_unload event handler to kld_unload_try

MFC r254266,254309: Add event handlers for module load and unload events

This is just a merge of EVENTHANDLER.9, the actual code chnage was merged earlier.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Fix a syntax nit (superfluous 'a').

Submitted by: Anders Hanssen <anders@rethink.no>

Reorganize the DESCRIPTION section, using .Bl lists to separate the
descriptions of items from each other and have related things appear
in the same nesting 'level'.

The .Fn function/macro, and bump document date.

Reviewed by: jkoshy

Update prototypes for eventhandler_register() and eventhandler_find_list()
to match the code.

Add a manual page for the kernel's EVENTHANDLER facility.

Reviewed by: ru