com32/chain: minor fixes and cleanups

com32/chain: minor fixes and cleanups

- make sure opt.maps implies opt.sect, and simplify
  conditions throughout the files
- move structure opt -> options.c
- adjust #includes
- omitted values in seg= and sect= are assumed 0
  in all cases now
- mangler_handover(): eax for gpt handover set only for
  iterated disks
- rename parse_args() -> opt_parse_args()
- rename mangler_common() -> mangler_init()
- add opt_set_defs()
- if mmapped sector would conflict with the file, turn
  off mmapping instead of bailing out
- update temporary chain.txt to reflect changes + other
  fixups

Signed-off-by: Michal Soltys <soltys@ziu.info>