MEDIUM: channel: Use CF_WRITE_EVENT instead of CF_WRITE_PARTIAL

Just like CF_READ_PARTIAL, CF_WRITE_PARTIAL is now merged with
CF_WRITE_EVENT. There a subtlety in sc_notify(). The "connect" event
(formely CF_WRITE_NULL) is now detected with
(CF_WRITE_EVENT + sc->state < SC_ST_EST).
5 files changed