panic: pmap_page_protect !PG_V

i used to get this panic every so often. it was always
preceeded by these acts (in this order, but, not
necessarily one after the other):

	- reboot machine.
	- run "startx" as me.
	- run "screen".
	- wait for reboot after panic.

it was almost always about 3-4 seconds after i started
screen.  however, this hasn't happened for at least 6
months on me.  i don't remember anything being fixed
that would stop this happening, it just did.