LDAP posixGroup capability

I've created a patch against 1.9.2 that makes it possible to allow only LDAP users belonging to a specific posixGroup to authenticate.

xwiki-core must be build with the attached patch and should replace the original xwiki-core-1.9.2.jar..
After that the DN for the posixGroup has to be added to xwiki.cfg:

1. • 1. only members of the following posix group will be verified in the LDAP
        xwiki.authentication.ldap.posix_group=cn=xwiki,ou=groups,dc=domain,dc=com

Regards,
Ruben