Declare that the tty used by TERMINAL does not handle underlining.
This is used to override the terminfo data, for certain terminals that
do not really do underlining, but say that they do. This function has
no effect if used on a non-tty terminal.
TERMINAL can be a terminal object, a frame or nil (meaning the
selected frame's terminal). This function always returns nil if
TERMINAL does not refer to a text terminal.
(fn &optional TERMINAL)