changing defvars in term/tty-colors.el to defconsts

[Dan Nicolaescu]

Any reason not to change these defvars:
color-name-rgb-alist
tty-standard-colors
tty-color-mode-alist
tty-defined-color-alist

to defconsts?

This will quite a few things from GC memory to pure memory.

Also (defvar msdos-color-values) seems unused in this file.