r19731: Modify the ldb_map infrustructure to always map from requested

r19731: Modify the ldb_map infrustructure to always map from requested
attributes to backend (remote) attributes.

We can't do a reverse mapping safely where the remote attribute may be
a source for multiple local attributes.  (We end up with the wrong
attributes returned).

In doing this, I've modified the samba3sam.js test to be more
realistic, and fixed some failures in the handling of primaryGroupID.

I've added a new (private) helper function ldb_msg_remove_element() to
avoid a double lookup of the element name.

I've also re-formatted many of the function headers, to fit into
standard editor widths.

Andrew Bartlett