kernel - Fix exec optimization race

* Fix an improper vm_page_unhold() in exec_map_page() which
  under heavy memory loads can cause a later assertion
  on m->hold_count == 0.

* Triggered every few days by bulk builds on pkgbox64.
kernel - Improve m->hold_count assertions, fix page-zeroing code bug

* Improve assertions on m->hold_count to provide more information.

* Fix a bug in vm_page_free_fromq_fast() where an assertion can get
  hit due to a SMP race.  This is only used by the page-zeroing code.