Add a hook_detect_ldapgroups for detecting groups.

I've written a patch that introduces a posibility for arbitrary ldap group
detection.

Our particular ldap setup couldn't be catered by the existing 3 strategies so
I implemented a 4th strategy: hook_detect_ldapgroups($ldap_connection, $user)
It enables anyone to write a custom module that implements the hook to cater
any weird scheme for detecting extra groups from LDAP.

I suspect it needs review, but it's a very simple patch with no side-effects,
so I hope it will be included in future releases.