s3:rpc_server: Implement the rpcd_* helper-end of the samba-dcerpc protocol

s3:rpc_server: Implement the rpcd_* helper-end of the samba-dcerpc protocol

This is the generic code that becomes the
template that all rpcd_* instances that
serve DCERPC can use to provide services to samba-dcerpcd.

The external entry point is:
rpc_worker_main() which takes an argc/argv list
and two functions:

get_interfaces() - List all interfaces that this server provides
get_servers() - Provide the RPC server implementations

Each rpcd_* service needs only to provide
the implementations of get_interfaces() and get_servers()
and call rpc_worker_main() from their main() function
to provide services that can be connected to from samba-dcerpcd.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>