Convert one instance of pfil hook callback missed in r254769.
Internalize pfil_hook_get().  There are no outside consumers of
this API, it is only safe for internal use and even the pfil(9)
man page says so in the BUGS section.

Reviewed by:	eri
pfil_hook_get() has been internalized in r254771 and is no longer
part of the API.  It wasn't safe for external use in any case.
Resolve the confusion between the head_list and the hook list.

The linked list of pfil hooks is changed to "chain" and this term
is applied consistently.  The head_list remains with "list" term.

Add KASSERT to vnet_pfil_uninit().

Update and extend comments.

Reviewed by:	eri (previous version)
ename PFIL_LIST_[UN]LOCK() to PFIL_HEADLIST_[UN]LOCK() to avoid
confusion with the pfil_head chain locking macros.