[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)
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc