r20986: Commit the prototype of the nss_info plugin interface.

r20986: Commit the prototype of the nss_info plugin interface.
This allows a provider to supply the homedirectory, etc...
attributes for a user without requiring support in core
winbindd code.  The idmap_ad.c module has been modified
to provide the idmap 'ad' library as well as the rfc2307 and sfu
"winbind nss info" support.

The SID/id mapping is working in idmap_ad but the nss_info
still has a few quirks that I'm in the process of resolving.
(This used to be commit aaec0115e2c96935499052d9a637a20c6445986e)