Support 24-bit direct colors on text terminals

Support 24-bit direct colors on text terminals

* src/term.c (init_tty): Use 24-bit terminal colors if corresponding
foreground and background functions are present in terminal type
definition.
* src/tparam.h: Define prototype for tigetstr.

* lisp/term/tty-colors.el (tty-color-define): Convert color palette
index to pixel value on 16.7M color terminals.
(tty-color-24bit): New function to convert color palette index to
pixel value on 16.7M color terminals.
(tty-color-desc): Don't approximate colors on 16.7M color terminals.
* lisp/term/xterm.el (xterm-register-default-colors): Define all named
TTY colors on 16.7M color terminals.

* doc/misc/efaq.texi (Colors on a TTY): Add instructions on how to
enable direct color TTY mode.
* etc/NEWS: Mention direct color TTY mode and point to FAQ.