Correct a very old and very obscure bug: vmspace_fork() calls
pmap_copy() if the mapping is VM_INHERIT_SHARE. Suppose the mapping
is also wired. vmspace_fork() clears the wiring attributes in the vm
map entry but pmap_copy() copies the PG_W attribute in the PTE. I
don't think this is catastrophic. It blocks pmap_remove_pages() from
destroying the mapping and corrupts the pmap's wiring count.

This revision fixes the problem by changing pmap_copy() to clear the
PG_W attribute.

Reviewed by: tegge@