Remove alternate username/pw auth file format

... in prefernce for a sasl file with optional username/pw.
Add -u option to store username/password with sasl

The SASL token is timing out, in a period somewhere between days
and months. Optionally save the username and password in authfile,
in a format maintained by cloudprint.py, as a means to reauthenticate
in that case.
Refresh XMPP authentication in process_jobs()

... if the username and password is known, to avoid problems with
the SASL token timing out.