r23995: Work to allow mimir's libnet code to be called from winbind.

r23995: Work to allow mimir's libnet code to be called from winbind.

We now setup a libnet_ctx for each domain.  We should then be able to
replace/merge some more of the winbind code with libnet calls,
referencing domain->libnet_ctx.

Andrew Bartlett
(This used to be commit bad2dc14d704be59300f619c84694c11620559e0)