[multi-sources] closes #1918120: drop behaviour leading to wrong result as demonstrated by the ticket
The formerly implemented behaviour doesn't seems to be useful,
nor to have a chance to be correct. At least no test case demonstrate
the opposite, so I've dropped it.
[session] enforce coherency of login/passwd couple values when no anonymous user (closes: #1910849)
None may be returned for both if anonymous connection is not
allowed or if an empty login is used in configuration
Raise ConfigurationError is anonymous login is non-ascii (convention)