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

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").