Consolidate auth

This PR is a first step toward saner authentication configuration in enstaller:

  - get_auth only returns auth information (if found). It never asks for user input, nor does it modify the .enstaller4rc
  - enpkg.main function now enforces authentication early (except for things like enpkg --version, etc...). This should fix issues like #144.

Authentication is still globally shared, this will be fixed through another PR.