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.1

@@ -6049,6 +6049,10 @@ Disable and enable focus reporting.
 These are set automatically if the
 .Em XT
 capability is present.
+.It Em \&Rect
+Tell
+.Nm
+that the terminal supports rectangle operations.
 .It Em \&Smol
 Enable the overline attribute.
 .It Em \&Smulx