Revise implementation of pmap_protect(9) in preparation for execute-only

Revise implementation of pmap_protect(9) in preparation for execute-only
support.  The current implementation doesn't handle the transition from
RWX to RW correctly.  Also generalize the pmap_write_protect() function
in recognition of the fact that execute permission, write permission,
and in the future read permission on executable pages, are handled by
separate bits.

ok deraadt@, mpi@