librpc:core: Add public functions to initialize endpoint servers

The dcesrv_init_registered_ep_servers() will be used by the S3 server to
initialize all registered endpoint servers (for embedded services), and
the dcesrv_init_ep_server() function will be used by the external
daemons to initialize the required ones.

As serveral S3 services may require to initialize another one before
itself (svcctl and eventlog for example require winreg) a boolean flag is
added to track the initialization status.

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