(xterm-modify-other-keys-terminal-list): New

(xterm-modify-other-keys-terminal-list): New
variable.
(xterm-turn-on-modify-other-keys): Only turn on modify-other-keys
if the selected frames is in
xterm-modify-other-keys-terminal-list.
(xterm-turn-off-modify-other-keys): Add an optional frame
parameter. Only turn off modify-other-keys if FRAME is in
xterm-modify-other-keys-terminal-list.
(xterm-remove-modify-other-keys): New function.
(terminal-init-xterm): Use it. Deal with delete-frame hook. Add
the selected frame to xterm-modify-other-keys-terminal-list.