support posix schema for user and group sync
1) plugin config ldif must contain pluginid, etc. during upgrade or it
will fail due to schema errors
2) posix winsync should have a lower precedence (25) than the default (50)
so that it will be run first
3) posix winsync should support the Winsync API v3 - the v2 functions are
just stubs for now - but the precedence cb is active