Corosync APIs return a standard set of CS_ERR_* error codes. Currently, we log these sometimes using cs_strerror() (which unhelpfully just stringifies the constant name) and sometimes with pcmk__cs_err_str() (our own more readable version).

Replace both of those with a new pcmk__corosync2rc() function that maps the CS_ERR_* codes to standard Pacemaker return codes (many can use standard system errnos, some can use existing Pacemaker codes, some may require new Pacemaker codes, and remaining rare ones could be lumped together into a generic "Corosync error"). Then we can just use pcmk_rc_str() on the result in logs.