Allow faster eventhandler dispatching by keeping pointers to handler lists.

[Ian Lepore]

There has been some talk lately of the kernel eventhandler mechanism
being inefficient due to holding a lock while walking a global list
doing strcmp() to find the right list of handlers.  I've posted a
phabricator review to alleviate that by allowing high-frequency events
to pre-define the event list and keep a pointer to it, to avoid the
name lookups.

https://reviews.freebsd.org/D12821

-- Ian