xterm: fix Backspace/Control+Backspace handling.

- make sure we're not impacted by initial PTY choice of erase character
- send a delete character on backspace press (that's what the Linux
  console does, or other terminals, like rxvt and gnome-terminal)
- send a custom code on Control+Backspace, different from Control+W and
  Control+H so those combinations can still be used (by VIM or Emacs for
  example).