[patch] idmap backend = external

[Matthew Mastracci]

This should be the final update for the patch - I fixed a bug in 
set_mapping where it wasn't passing the SID correctly.

AFAIK, the patch should compile against the latest code on the release 
3.0 branch, except that the allocate_rid function can be deleted (it no 
longer seems to appear in the idmap interface).

> Command          Parameter(s)              Return Values (1 per line)
> init             interface_version      -> no return values
> set_mapping      sid uid/gid id_type    -> no return values
> allocate_rid     rid_type               -> rid
> allocate_id      id_type                -> id
> get_sid_from_id  id id_type             -> sid
> get_id_from_sid  sid                    -> id id_type

Matt.