Change the flags that are set for an evbuffer by adding more.
@param buffer the evbuffer that the callback is watching.
@param cb the callback whose status we want to change.
@param flags One or more EVBUFFER_FLAG_* options
@return 0 on success, -1 on failure.
Change the flags that are set for an evbuffer by adding more.
@param buffer the evbuffer that the callback is watching. @param cb the callback whose status we want to change. @param flags One or more EVBUFFER_FLAG_* options @return 0 on success, -1 on failure.