(color-name-rgb-alist): Use 16-bit RGB values instead of 8-bit, for

(color-name-rgb-alist): Use 16-bit RGB values instead of 8-bit, for
consistency with the rest of emacs.
(tty-color-canonicalize): Only copy COLOR if we need to change it.
(tty-color-approximate): Scale values from `color-name-rgb-alist'
appropriately.  Return the whole color description, rather than just the
RGB values.
(tty-color-standard-values): New function.  Code mostly taken from
`tty-color-translate'.
(tty-color-translate, tty-color-values): Use `tty-color-desc' to do all
the work.
(tty-color-desc): Do color lookup here instead of calling
`tty-color-translate' and `tty-color-by-index'; this is now the main
place to do it.