r8037: a fairly major update to the internals of ldb. Changes are:

r8037: a fairly major update to the internals of ldb. Changes are:

 - moved the knowledge of attribute types out of ldb_tdb and into the
   generic ldb code. This allows the ldb_match() message match logic
   to be generic, so it can be used by other backend

 - added the generic ability to load attribute handlers, for
   canonicalisation, compare, ldif read and ldif write. In the future
   this will be used by the schema module to allow us to correctly
   obey the attributetype schema elements

 - added attribute handlers for some of the core ldap attribute types,
   Integer, DirectoryString, DN, ObjectClass etc

 - added automatic registration of attribute handlers for well-known
   attribute names 'cn', 'dc', 'dn', 'ou' and 'objectClass'

 - converted the objectSid special handlers for Samba to the new system

 - added more correct handling of indexing in tdb backend based on the
   attribute canonicalisation function

 - added generic support for subclasses, moving it out of the tdb
   backend. This will be used in future by the schema module

 - fixed several bugs in the dn_explode code. It still needs more
   work, but doesn't corrupt ldb dbs any more.