24-bit direct color support (patch by Fengguang Wu)

Support directly setting RGB fg/bg colors via ISO-8613-3 24-bit
ANSI color escapes:

  ESC[38;2;<r>;<g>;<b>m Select RGB foreground color
  ESC[48;2;<r>;<g>;<b>m Select RGB background color

The killer applications for me are vim in tmux. It'll not only modernize
their look and feeling, but also bring more eye friendly color schemes.
Very helpful for long time programmers.

To avoid memory overheads and keep the patch non-intrusive, it takes the
approach to adapt the nearest color in an hidden 6x6x4 (88-color mode)
or 7x7x5 (256-color mode) color cube to the new 24-bit RGB color.

The pros/cons are:

+) least memory footprint (close to 0)
   comparing to konsole, gnome-terminal etc. real 24-bit arrays

+) exact colors and excellent user feelings
   comparing to xterm, mlterm, etc. approximating to 256 palette

+) usable in both the existing 88/256-color modes

   Most vim GUI color schemes show up the same as gvim in rxvt-unicode's
   88-color mode, not to mention the 256-color mode. Typical applications
   only use one or two dozens of colors at the same time.

-) may not be able to show 2+ close 24-bit colors

   RGB colors close to each other will likely fall into the same slot in
   the 6x6x4 or 7x7x5 color cube. If necessary, it could be improved
   effectively by implementing some collision avoidance logic, trying to
   find empty/eldest slot in the +1/-1 r/g/b indices (ie. 3-8 neighbors).

The CPU overheads of map_rgb24_color() look ignorable: I feel no
perceptible slow down when doing vim operations in 24-bit color mode.

A micro benchmark running a test script from [1]:

% time (for i in {1..100}; do 24-bit-color.sh; done)

vanilla rxvt-unicode
====================
  2.42s user 1.88s system 31% cpu 13.555 total
  2.59s user 1.74s system 31% cpu 13.615 total
  2.46s user 1.85s system 31% cpu 13.631 total

THIS PATCH (adapt hidden color cube to 24-bit)
==============================================
  2.33s user 1.97s system 31% cpu 13.598 total
  2.46s user 1.89s system 31% cpu 13.613 total
  2.51s user 1.82s system 31% cpu 13.556 total

https://github.com/spudowiar/rxvt-unicode (real 24-bit array)
=============================================================
  2.61s user 1.75s system 31% cpu 13.721 total
  2.48s user 1.82s system 31% cpu 13.566 total
  2.60s user 1.76s system 31% cpu 13.631 total

USE_256_COLORS is defined in all the above rxvt-unicode builds.

References:

[1] True Colour (16 million colours) support in various terminal
    applications and terminals
    https://gist.github.com/XVilka/8346728

[2] https://en.wikipedia.org/wiki/ANSI_escape_code#Colors