patch 9.0.0007: no support for double, dotted and dashed underlines

Problem:    No support for double, dotted and dashed underlines.
Solution:   Add the termcap entries and highlight modes. (closes #9553)
diff --git a/src/gui.c b/src/gui.c
index e708a27..d7e768f 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -2501,6 +2501,8 @@
     if (hl_mask_todo & HL_UNDERCURL)
 	draw_flags |= DRAW_UNDERC;
 
+    // TODO: HL_UNDERDOUBLE, HL_UNDERDOTTED, HL_UNDERDASHED
+
     // Do we strikethrough the text?
     if (hl_mask_todo & HL_STRIKETHROUGH)
 	draw_flags |= DRAW_STRIKE;