Purification, *PURE=T with PURE=NIL, and other lossage

[HIC at MIT-MC (Howard I. Cannon)]

LOSSAGE!

A) With PURE=nil and *PURE=T, the UUO handler tries to clobber UUO's in pure
   code!!!!

B) If you purify after loading one file but before loading a second file, then
   pointers into freespace are left pointing into purified pages.

I think the whole mechanism needs some review and thus I am not going to
touch it until we get to talk more.  The purification mechanisms seem somewhat
ill-defined and this is what is causing these random screws.

--Howard