Cleaned up comments in Atari 8-bit headers.
Internal keycodes (POKEY's KBCODE) were already #defined in atari.h,
so didn't need a whole new set in _pokey.h.
Relocated register #define'd values outside of the structs,
improved comment format, expanded & corrected some things.
h/t Trevin Beattie (https://user.xmission.com/~trevin/) for the PIA
register descriptions.
Add #defines for certain registers' values.
Also add #defines for internal keyboard codes (unrelated to ATASCII;
e.g. [Q] = 47, [W] = 46, [Shift] adds 64, etc), as seen in KBCODE.
