fix #H7677 - guard placed twice at <0,0>

fix #H7677 - guard placed twice at <0,0>

"Placing monster over another?" warning was triggered for vault guard
by an earlier change which made m_detach() stop removing monsters at
<0,*> from level.monsters[][].  So one guard would replace another at
<0,0> for however many guards were created, and memory for all but
the last one would be lost.

This involved a lot of flailing about and the patch includes various
things would could have been discarded.  One or two extended monster
sanity checks are included, plus a couple of debugpline()'s for
tracking guard movement.