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").

tmux.h

@@ -452,6 +452,7 @@ enum tty_code_code {
 	TTYC_MS,
 	TTYC_OL,
 	TTYC_OP,
+	TTYC_RECT,
 	TTYC_REV,
 	TTYC_RGB,
 	TTYC_RI,