- Add interrupt moderation support by only using TIMER intr; IM is not

- Add interrupt moderation support by only using TIMER intr; IM is not
  possible when RX or TX intr is enabled.  With this change, my nfe(4)
  can TX at line speed finally (mtu 1500).
- Add per iface sysctl node hw.nfeX.imtime to enable, disable and set
  interrupt moderation time.  Set it to -1 will disable IM; 64-128
  seems to be a reasonable value range.  This sysctl variable is _not_
  on by default, and can be tuned when the iface is running.
- Nuke global sysctl nodes, make them per iface.

# We may consider using TIMER intr under all situations, but with larger
# imtime when IM is not enabled, since for certain type of nfe(4), TX
# interrupts may never come.