emacs: Normalize ido-mode behavior for tab/backtab.

I'm used to <tab> always completing the available common prefix (which
ido shows in square brackets. <space> does this by default, this forces
<tab> to do the same. Failing a completion, ido should also cycle
through matches in order, which is mirrored then by <backtab>.
emacs: Add smex initialization and bindings.