Fixes for extended keys: 1) allow C-x and C-X to be bound separately

Fixes for extended keys: 1) allow C-x and C-X to be bound separately
since some terminals report them differently 2) use the "backspace"
option to translate backspace 3) map ctrl which are have the ctrl
implied (such as C-x) properly when the terminal reports both the key
and the modifier.

Note that any key bindings for C-X where C-x is meant must now be
changed.