s4-kdc: Add a new kpasswd service implementation

This function is intended to be be passed to kdc_add_socket(). The
function kpasswd_handle_request() which is called by kpasswd_process()
is Kerberos implementation specific and should be implemented in a
kpasswd-service-<kerberos flavour>.c file.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
