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. (In the
future, when we support "hardware dirty bit management", using the value
from the pmap_load() rather than the pmap_load_clear() would have actually
been an error because the dirty bit could potentially change between the
pmap_load() and the pmap_load_clear().)

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().

MFC after: 2 weeks