Function: set-face-underline

Specify whether face FACE is underlined.
UNDERLINE nil means FACE explicitly doesn't underline.
UNDERLINE t means FACE underlines with its foreground color.
If UNDERLINE is a string, underline with that color.

UNDERLINE may also be a list of the form (:color COLOR :style STYLE),
where COLOR is a string or `foreground-color', and STYLE is either
`line' or `wave'. :color may be omitted, which means to use the
foreground color. :style may be omitted, which means to use a line.

FRAME nil or not specified means change face on all frames.
Use `set-face-attribute' to ``unspecify'' underlining.