Refactor: fencer: use common reply handling for T_STONITH_NOTIFY

Previously, handle_request() called pcmkipc_send_ack() directly for
T_STONITH_NOTIFY, rather than let it fall through to stonith_send_reply(). With
recent refactors, the equivalent can be done with pcmkipc_create_ack() and
letting it fall through to the same reply handling as CRM_OP_REGISTER.

Now, any reply is always sent at the end of handle_request().