user_modify should use radius_map2vp to get values for updates, and use berval structs

[arr2036]

Currently it's not possible to insert binary data into LDAP because the user_modify function just works with strings.

It would be better to use radius_map2vp to gather the attribute values so the RHS of the map can be the standard set of value formats used in the rest of the server.

Octets type values could then be represented as bervals.