rename sTHX to PERL_MEMORY_DEBUG_HEADER_SIZE

rename sTHX to PERL_MEMORY_DEBUG_HEADER_SIZE

Originally this macro evaluated to the size of the THX pointer;
this was used with the memory pool facility to store the context of the
caller of the malloc() for debugging. Later, the header of the malloc()
stored more than just the context, so sTHX became a mismoner.

Rename it to something less misleading.