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.

1 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 73085f626b1..143e4655def 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -77,6 +77,12 @@ modern init systems such as systemd, which manage many of the traditional
 aspects of daemon behavior themselves.  '--old-daemon' is now an alias
 for '--daemon'.
 
++++
+** Emacs now supports 24-bit colors on capable text terminals
+Terminal is automatically initialized to use 24-bit colors if the
+required capabilities are found in terminfo.  See the FAQ node
+"Colors on a TTY" for more information.
+
 
 * Changes in Emacs 26.1