Make worker-control's _chan_buf_send implementation to use resource event loop

[prajnoha]

The _chan_buf_send function which is part of the worker-control still uses its own loop to send the data:

sid/src/resource/worker-control.c

Lines 883 to 896 in 0fecd48

	for (;;) {
	n = comms_unix_send(chan->fd, &byte, sizeof(byte), data_spec->ext.socket.fd_pass);
	if (n < 0) {
	if (n == -EAGAIN || n == -EINTR)
	continue;
	log_error_errno(ID(chan->owner), n, "Failed to send ancillary data on channel %s", chan->spec->id);
	r = n;
	goto out;
	}
	break;
	}

We should get rid of this loop and make this a part of the resource event loop like it is in_chan_buf_recv. For this, we also need sid_resource_create_io_event_source to accept a new arg - a bit mask of events to watch for as currently it implicitly uses EPOLLIN only.