Fixed handling the [Abort] button in installer (inst_user_first)

- Uses are now asked to confirm that they really really want to Abort
- bnc#886662
- I've also fixed UsersSimple.Transliterate not to return a trailing newline
Since you're factoring out the function, note that it is badly designed. Proposing the login is a complex little operation that should be kept separate from the UI (which can remain inlined at the caller).
Just imagine (or even write, *shudder*!) its tests.