editors/emacs: backport 24-bit color fix for tty mode

libncurses in base system only supports termcap(5). RGB has 3 colors
but termcap(5) supports at most 2 arguments e.g.,

$ export TERMCAP="${TERM}:Co#0x1000000:AF=\E[38;2;%d;%d;%dm:AB=\E[48;2;%d;%d;%dm:tc=${TERM}:"
$ tput AB 1 1 1
tput: too many arguments (3) for capability `AB'

To work around the limitation make Emacs check if environ(7) has
COLORTERM=truecolor some terminal emulators define by default e.g.,

• x11-toolkits/vte3 (gnome-terminal, sakura, ...)
• x11/alacritty
• x11/aminal
• x11/kitty (bug 233010)
• x11/konsole
• x11/qterminal

while others require defining COLORTERM=truecolor manually e.g.,

• x11/cool-retro-term
• x11/sterm
• x11/xterm

PR: 247248
Approved by: jrm
Obtained from: upstream