Reinstate zone allocator from the original doom source code.
Add OVERLAY macros to z_zone.c
Fix Makefile_xmos, add --no-overlay-data option.
Reformatting.
Remove I_GetTime indirect calls.
Change thinker function member from a union to a function pointer.
Remove use of function pointers for thinkers.

Instead use an enum to specify which function to call.
Remove unused member.
Move prototypes for action functions into headers.
Remove use of function pointers for actions.

Instead use a enum to specify which action to perform.
Remove unused version control IDs.
Const correctness.
Remove unused variable.
Use switch statements instead of indirect calls for demo sequences.
Remove unused variables.
Make variables static where possible, give enums names.
Avoid indirect call in M_DoScreenShot.
Remove unused lookup table.
Remove DJGPP define.
Extract some of M_Responder into separate methods.