LDAP protocol version: PluginSetting / "auto"

The LDAP protocol needs to be configurable.

E.g., if a server only supports version 3,
ldap_set_option($ldap_conn, LDAP_OPT_PROTOCOL_VERSION, 3);
would be required (maybe depending on php.ini).

I'd say to have a PluginSetting for this, which defaults to "auto". In this case, we would try binding in the following order:
1. Without setting LDAP_OPT_PROTOCOL_VERSION
2. Setting LDAP_OPT_PROTOCOL_VERSION=3
3. Setting LDAP_OPT_PROTOCOL_VERSION=2

See http://forums.b2evolution.net//viewtopic.php?p=53690#53690