bug#62994: Support styled underlines on TTY frames

[Mohsin Kaleem <mohkale@kisara.moe>]

Hi,

Support for styled underlines double, wave, dotted and dashed is a
relatively recent inclusion for newer terminal emulators. It's exposed
through either the Smulx or Su termcap (the former being an escape
sequence and the latter just a flag). Furthermore. despite not having a
dedicated termcap, these extensions also support colored underlines
through a separate escape sequence; making it possible to have an
underline with a different color to the foreground.

I'd like to add support for these to Emacs. Looking on the mailing list
it seems there's some prior efforts for this but none seem to have had
any traction. I've been working on a patch for these and will submit it
in reply to this message chain.

For testing I've been using kitty. If building with this patch series
you can view the underlines in the scratch buffer like so:

  ./src/emacs -nw -q --eval '(set-face-attribute (quote font-lock-comment-delimiter-face) nil :underline (list :style (quote wave) :color "blue"))'

-- 
Mohsin Kaleem