FreeBSD only ships with /etc/termcap. However, termcap is limited to 2
arguments while RGB colors require 3. To support RGB add a fallback via
COLORTERM, using semicolon version for better compatibility.

https://unix.stackexchange.com/questions/512845/emacs-with-24bit-terminal-support-on-freebsd
https://gist.github.com/XVilka/8346728#true-color-detection
https://github.com/alacritty/alacritty/issues/1485

Test case:
$ pkg install emacs-devel-nox alacritty libxkbcommon mesa-dri dejavu
$ alacritty
$ env -u COLORTERM emacs -nw -Q -f list-colors-display
$ env COLORTERM=rxvt emacs -nw -Q -f list-colors-display
$ env COLORTERM=truecolor emacs -nw -Q -f list-colors-display