bug#45230: 27.1; Fix support for wavy underlines on terminal mode

[Eli Zaretskii]

> From: Víctor Cuadrado Juan <me@viccuad.me>
> Date: Sun, 13 Dec 2020 23:29:15 +0100
> 
> Support for (:underline (:style wave) when on terminal move is missing.
> See src/xfaces.c, and the line:
> 
>     return false;               /* ttys can't use wave underlines */
> 
> Since ~3 years, support for curly underscores and colored underscores is
> present in several terminal emulators. At least:
> - Kitty: 
> https://github.com/kovidgoyal/kitty/blob/master/protocol-extensions.asciidoc
> - VTE
> - iTerm2: https://gitlab.com/gnachman/iterm2/-/issues/6382

Patches to support this in Emacs are welcome (although it's a pity
that these features aren't based on terminfo capabilities, AFAICT).