MFC r349323, r349442, r349866, r349975

MFC r349323, r349442, r349866, r349975
  pmap_enter_quick_locked() never replaces a valid mapping, so it need not
  perform a TLB invalidation.  A barrier suffices.  (See r343876.)

  Add a comment to pmap_enter_quick_locked() in order to highlight the fact
  that it does not replace valid mappings.

  Correct a typo in one of pmap_enter()'s comments.

  Introduce pmap_clear(), which zeroes a page table entry, and use it,
  instead of pmap_load_clear(), in places where we don't care about the page
  table entry's prior contents.

  Eliminate an unnecessary pmap_load() from pmap_remove_all().  Instead, use
  the value returned by the pmap_load_clear() on the very next line.

  A KASSERT() in pmap_enter(), which originated in the amd64 pmap, was meant
  to check the value returned by the pmap_load_clear() on the previous
  line.  However, we were ignoring the value returned by the
  pmap_load_clear(), and so the KASSERT() was not serving its intended
  purpose.  Use the value returned by the pmap_load_clear() in the
  KASSERT().