fix typo & factory for single-val attributes

Before, you were able to pick any attribute, even multi-valued ones, for settings which were supposed to be limited to single-value attributes, such as rDN.
store binary attr properly in the LDAPUserFolder

Binary flag was not being passed, and thus was always false.
store blank schema property fields as empty strings, not None

An empty optional form field is returned as None, but this was being stored in
the LDAPUserFolder object (and also the export from portal_setup) as the
literal string 'None'.

This change makes plone.app.ldap match the behavior of the ZMI (LDAPUserFolder).
Merge other fixup branches into AD_fixes