s3:include: let nameserv.h be useable on its own

s3:include: let nameserv.h be useable on its own

A lot of stuff is private to nmbd and can
be moved from nameserv.h.

This allows move required types from smb.h to
nameserv.h, so that this can be standalone.
Including it from smb.h is not a huge problem
as nmbd internals are gone from nameserv.h.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15620

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>