Move string manipulation functions from externs.h into strutil.h.

Also fixes a bunch of headers to include what they use. This is a
continuation of the work to clean up core modules like the allocator,
hash tables, the memory checker, etc so they don't depend on anything
other than other core modules and config.h, confmagic.h, mushtypes.h,
and options.h.

git-svn-id: https://pennmush.googlecode.com/svn/trunk@1779 ba372814-4f39-11de-9ad6-1127a62b9fcd