bug#69598: 29.2; colour support based on $TERM value not terminfo database

[chohag--- via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii writes:
>
>   ** Emacs can support 24-bit color TTY without terminfo database.
>   If your text-mode terminal supports 24-bit true color, but your system
>   lacks the terminfo database, you can instruct Emacs to support 24-bit
>   true color by setting 'COLORTERM=truecolor' in the environment.  This is
>   useful on systems such as FreeBSD which ships only with "etc/termcap".
>
>   *** Emacs will now use 24-bit colors on terminals that support "Tc" capability.
>   This is in addition to previously-supported ways of discovering 24-bit
>   color support: either via the "RGB" or "setf24" capabilities, or if
>   the 'COLORTERM' environment variable is set to the value "truecolor".
>
> Did you try the COLORTERM=truecolor setting?

I did now, in a new non-xterm terminal which does display all colours
in emacs if $TERM is xterm-direct, and no joy (only lists 8 colours).

I also performed this sequence on a linux (Debian 6.1.67-1 (2023-12-12))
box in xterm (379) now that my cat has vacated it:

        $ export TERM=xterm-direct
        $ emacs -nw                   # Version 28.2
        $ echo 'fancy|Fancy Term,use=xterm-direct,' > fancy.info
        $ tic fancy.info
        $ export TERM=fancy
        $ emacs -nw

In the first emacs, list-colors-display listed (I presume) 256
colours. Certainly a lot and with the X names (it does not name
them nicely if the terminal reports 256 colours). In the second it
listed 8. Emacs has no configuration on that box (it's for compiling).

I recall testing the RGB capability late last night despite not
seeing it documented anywhere and that did not work. I shall
experiment with the Tc capability and RGB more carefully and see
what effect they have nevertheless I think the above sequence now
repeated on two distinct operating systems is quite telling.

Both systems have little or no customisation beyond the base image
apart from installed packages. Certainly nothing that should affect
terminfo (apart from running tic).

Cheers

Matthew