Initial support for XTerm 256-color mode.

You can now use CSI [ 38 ; 5 ; X m to set the foreground colour, and
CSI [ 48 ; 5 ; X m to set the background colour from the palette and use
any of the 256 standard colours, any of the 16 Commodore 64 colours (start
at 256), or the two Atari colors (after the C64 colours).  In addition,
you can use the palette redefinition commands to extend the palette past
those entries.

NOTE: Since this is only CTerm so far, using SyncTERM and UIFC features
will lose the palette blocks for the characters, and they will revert to
their current 16-color value.  Further, I haven't testing how this interacts
with alternat fonts, blinking, etc yet.

But it looks cool!