r20289: IDMAP is part of winbind but not the main process.

r20289: IDMAP is part of winbind but not the main process.
Make sure we route all request to remote DCs via the main process
so that IDMAP can correctly reuse DC connections and use the
async interface.

This fixes also idmap_nss so that it is able to resolve local
group names (requires patch on the samba dc earlier committed
to SAMBA_3_0 to make it resolve both the mapped and the unmapped
name).

Simo.
(This used to be commit 4297510f22c3fd60afd062e3c5eb142be2122b16)