altq: Add byte based limit and counter

- This avoids having too much mbufs sitting on the send queue for TSO
  capable devices.  Even by default, DragonFly has already limited TSO
  burst to at most 4 TCP segments, for TSO capable devices, there still
  could be 4 times mbufs sitting on the send queue compared with non-TSO
  capable devices.
- This paves way for the AQMs, which require send queue byte counter,
  e.g. CoDel.

For ethernet devices, the byte based limit is (1514 x max_packets).

For other devices, e.g. pseudo devices, the byte based limit is
(MCLBYTES x max_packets).
socket: Prioritiries rcvd message