r20184: change ldb_attrib_handler into ldb_schema_attribute, which has a pointer

r20184: change ldb_attrib_handler into ldb_schema_attribute, which has a pointer
to a ldb_schema_syntax struct.

the default attribute handler is now registered dynamicly as "*"
attribute, instead of having its own code path.

ldb_schema_attribute's can be added to the ldb_schema given a
ldb_schema_syntax struct or the syntax name

we may also need to introduce a ldb_schema_matching_rule,
and add a pointer to a default ldb_schema_matching_rule
in the ldb_schema_syntax.

metze
(This used to be commit b97b8f5dcbce006f005e53ca79df3330e62f117b)