nm: Pass back the username auth-dialog runs as to access ssh-agent socket

nm: Pass back the username auth-dialog runs as to access ssh-agent socket

This ensures we access the socket as user who NM ran the auth-dialog for,
especially for system-wide connections where the connection does not
mention a user.

We also make sure we don't use the cached socket and user of a previous
connection attempt, because system-wide connections might be used by
different users.