s3:rpc_server: Add samba-dcerpcd

s3:rpc_server: Add samba-dcerpcd

Central dispatcher for incoming RPC requests, supported by helpers
that implement RPC services.

Upon startup, it asks all helpers which interfaces and endpoints to
listen on so it doesn't interfere with the samba binary when we're
configured as an Active Directory Domain Controller, then samba-dcerpcd
opens the relevant sockets. Once clients connect, start required helpers
and tell them to shut down once idle for a while.

Can be started as a full standalone daemon without smbd involved or as
a helper daemon started on demand by smbd or winbind or other local
processes trying to connect to a named pipe based RPC service.

NB. To start as a standalone daemon the smb.conf [global] option
"rpc start on demand helpers = false" must be set.
By default "rpc start on demand helpers = true"
in order to allow upgrades without needing an smb.conf change.

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>