Add Smulx to terminfo

A couple of other terminals use it for styled underlines, in
contravention of the spec. But, in the interests of wider
interoperability add it to kitty as well.

Fixes #3725

terminfo/kitty.terminfo

@@ -16,6 +16,7 @@ xterm-kitty|KovIdTTY,
 	it#8,
 	lines#24,
 	pairs#32767,
+	Smulx=\E[4:%p1%dm,
 	acsc=++\,\,--..00``aaffgghhiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
 	bel=^G,
 	bold=\E[1m,