Refactor: daemons: Use pcmk__request_t for LRMD_OP_RSC_REG.

Handling this message type means we also need to send out a generic
notify message after sending the reply. For the moment, the rc value
being passed to execd_send_generic_notify doesn't matter because
execd_process_rsc_register doesn't return anything (previously, it only
ever returned pcmk_ok).