patch 8.2.0863: cannot set a separate color for underline/undercurl

Problem:    Cannot set a separate color for underline/undercurl.
Solution:   Add the t_AU and t_8u termcap codes. (Timur Celik, closes #6011)
diff --git a/src/structs.h b/src/structs.h
index eb4f60e..f67aba3 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1068,9 +1068,11 @@
 	    // These colors need to be > 8 bits to hold 256.
 	    short_u	    fg_color;	// foreground color number
 	    short_u	    bg_color;	// background color number
+	    short_u	    ul_color;	// underline color number
 # ifdef FEAT_TERMGUICOLORS
 	    guicolor_T	    fg_rgb;	// foreground color RGB
 	    guicolor_T	    bg_rgb;	// background color RGB
+	    guicolor_T	    ul_rgb;	// underline color RGB
 # endif
 	} cterm;
 # ifdef FEAT_GUI