Low: libcrmcommon: Add EAGAIN as a return code from crm_ipc_read.

If qb_ipcc_event_recv returns -EAGAIN, we should pass that on to the
caller instead of converting it to -ENOMSG. This allows the caller to
distinguish cases where it should try again from other error conditions.

In pcmk__send_ipc_request, just continue returning pcmk_rc_ok in this
case for the moment. That will be changed in a later patch.