- 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.