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

@@ -451,6 +451,7 @@ enum tty_code_code {
 	TTYC_MS,
 	TTYC_OL,
 	TTYC_OP,
+	TTYC_RECT,
 	TTYC_REV,
 	TTYC_RGB,
 	TTYC_RI,