Merge pull request #14 from groessler/something_to_pull

Use symbolic names for all CIO commands.
Avoid GCC "unconst" warning.
Have __APPLE2ENH__ imply __APPLE2__ and __ATARIXL__ imply __ATARI__.
Avoid macro redefinition of CH_DEL for apple2enh.

I'm wondering why MS VC++ didn't complain about this...
Simply define CH_DEL exactly then when there none yet.