Remove externs.h dependency from memcheck/mymalloc.

Part of the continued work to make it so the entire game doesn't depend
on externs.h to be included. list_mem_check() is a little clunkier to
use now unfortunately, but this was probably the least bad alternative.

git-svn-id: https://pennmush.googlecode.com/svn/trunk@1781 ba372814-4f39-11de-9ad6-1127a62b9fcd