evch_usrunsubscribe
evch_usrunsubscribe(ctl->chp, NULL, 0);
evch_usrunsubscribe(ctl->chp, sid, 0);
evch_usrunsubscribe(ctl->chp, NULL, EVCH_SUB_KEEP);
extern void evch_usrunsubscribe(evchan_t *bp, const char *subid, uint32_t flag);