s3:mdssvc: add noindex backend

s3:mdssvc: add noindex backend

Add a new default backend that, while allowing mdsvc RPC and search queries from
clients, always returns no results.

Shares using this backend will behave the same way as shares on a macOS SMB
server where indexing is disabled.

This change will later also allow us to compile the Spotlight RPC service by
default which is a big step in the direction of adding tests to CI.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>