I believe UVM is clean now and no managed pages should be in use anymore

I believe UVM is clean now and no managed pages should be in use anymore
when we call pmap_release().  Turn debug code into printf and remove the code
that tried to mop up.  Override Debugger() such that it inserts a trap
instruction directly into the code such that we get useful backtraces.

This way I don't need to worry about pmap_release() removing pv entries when
making this pmap mpsafe.