Three changes to fix problems with xterm in VT340 mode, reported by Thomas

Sattler.

1) Do not include the DECSLRM or DECFRA features for xterm; they will be added
   instead if secondary DA responds as VT420 (this happens already).

2) Set or reset the individual flags after terminal-overrides is applied, so
   the user can properly disable them.

3) Add a capability for DECFRA ("Rect").

tty-features.c

@@ -218,9 +218,13 @@ static const struct tty_feature tty_feature_margins = {
 };
 
 /* Terminal supports DECFRA rectangle fill. */
+static const char *tty_feature_rectfill_capabilities[] = {
+	"Rect",
+	NULL
+};
 static const struct tty_feature tty_feature_rectfill = {
 	"rectfill",
-	NULL,
+	tty_feature_rectfill_capabilities,
 	TERM_DECFRA
 };
 
@@ -351,8 +355,13 @@ tty_default_features(int *feat, const char *name, u_int version)
 			      ",cstyle,extkeys,margins,sync"
 		},
 		{ .name = "XTerm",
+		  /*
+		   * xterm also supports DECSLRM and DECFRA, but they can be
+		   * disabled so not set it here - they will be added if
+		   * secondary DA shows VT420.
+		   */
 		  .features = TTY_FEATURES_BASE_MODERN_XTERM
-			      ",ccolour,cstyle,extkeys,focus,margins,rectfill"
+			      ",ccolour,cstyle,extkeys,focus"
 		}
 	};
 	u_int	i;