credential : always using the system keystore

Supersedes !1308 (closed)

• The system keystore is now used before trying the Guest account.

Abandoned:

• Small refactor of guest account handling
• Add a way to force a user interaction
• Propagate the logged user to LibVLC

We will need a LibVLC and Core API to remove keystores entries instead of the previous 3 points, in a future MR.