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
    (cherry picked from commit 4f1a19a9feab89d0cb5e8cce7916b78cee2cfe12)
    (cherry picked from commit 01ef3aac2bbd8521df14526ef2b548bb8785cee7)