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
`@login_modified = true` is set ONLY if user fills the username manually

`@login_modified = false` is (re)set if user deletes what they have filled there

Then this function starts proposing login again