Rename {_,}pmap_unwire_pte_hold() to {_,}pmap_unwire_ptp() and update the
comment describing them. Both the function names and the comment had grown
stale. Quite some time has passed since these pmap implementations last
used the page's hold count to track the number of valid mapping within a
page table page. Also, returning TRUE from pmap_unwire_ptp() rather than
_pmap_unwire_ptp() eliminates a few instructions from callers like
pmap_enter_quick_locked() where pmap_unwire_ptp()'s return value is used
directly by a conditional statement.