Switch user switch hotkey to Ctrl-Alt-O:

- It's right next to Ctrl-Alt-L on most standard keyboard layouts
  (QWERTY, QWERTZ, AZERTY) so easy to use as a double key combo
  Ctrl-Alt-LO (rather than Ctrl-Alt-LL) to lock and switch.

- It prevents accidental double-taps on L from invoking the user switch
  (annoys some users).