subr_pctrie: add leaf callbacks to pctrie_reclaim

PCTRIE_RECLAIM frees all the interior nodes in a pctrie, but is little
used because most trie-destroyers want to free leaves of the tree
too. Add PCTRIE_RECLAIM_CALLBACK, with two extra arguments, a callback
function and an auxiliary argument, that is invoked on every non-NULL
leaf in the tree as the tree is destroyed.

Reviewed by: rlibby, kib (previous version)
Differential Revision: https://reviews.freebsd.org/D45565