patch 8.2.0906: when setting 'termguicolors' SpellBad is no longer red

Problem:    When setting 'termguicolors' SpellBad is no longer red.
Solution:   Only use the RGB guisp color for cterm when using the "underline"
            or "undercurl" attributes to avoid the background color to be
            cleared. Also make t_8u empty when the termresponse indicates a
            real xterm. (closes #6207)

src/highlight.c

@@ -2929,7 +2929,12 @@ set_hl_attr(
 #  endif
 	at_en.ae_u.cterm.fg_rgb = GUI_MCH_GET_RGB2(sgp->sg_gui_fg);
 	at_en.ae_u.cterm.bg_rgb = GUI_MCH_GET_RGB2(sgp->sg_gui_bg);
+	// Only use the underline/undercurl color when used, it may clear the
+	// background color if not supported.
+	if (sgp->sg_cterm & (HL_UNDERLINE | HL_UNDERCURL))
 	    at_en.ae_u.cterm.ul_rgb = GUI_MCH_GET_RGB2(sgp->sg_gui_sp);
+	else
+	    at_en.ae_u.cterm.ul_rgb = INVALCOLOR;
 	if (at_en.ae_u.cterm.fg_rgb == INVALCOLOR
 		&& at_en.ae_u.cterm.bg_rgb == INVALCOLOR)
 	{

src/term.c

@@ -4819,6 +4819,11 @@ not_enough:
 			if (version < 95)
 			    is_not_xterm = TRUE;
 
+			// With the real Xterm setting the underline RGB color
+			// clears the background color, disable "t_8u".
+			if (!is_not_xterm && *T_8U != NUL)
+			    T_8U = empty_option;
+
 			// Only request the cursor style if t_SH and t_RS are
 			// set. Only supported properly by xterm since version
 			// 279 (otherwise it returns 0x18).

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    906,
 /**/
     905,
 /**/