Add `xterm-extra-capabilities' defcustom for terminals where the xterm capabilities...

Add `xterm-extra-capabilities' defcustom for terminals where the xterm capabilities query is not needed or wanted.

* term/xterm.el (xterm): Add defgroup.
(xterm-extra-capabilities): Add defcustom to supply known xterm
capabilities, skip querying them, or query them (default).
(terminal-init-xterm): Use it.
(terminal-init-xterm-modify-other-keys): New function to set up
modifyOtherKeys support to simplify `terminal-init-xterm'.