Enable net.isr.enable by default, causing "delivery to completion"
(direct dispatch) in interrupt threads when the netisr in question
isn't already active. If a netisr is already active, or direct
dispatch is already in progress, we queue the packet for later
delivery. Previously, this option was disabled by default. I have
measured 20%+ performance improvements in IP packet forwarding with
this enabled.

Please report any problems ASAP, especially relating to stack depth or
out-of-order packet processing.

Discussed with: jlemon, peter
Sponsored by: DARPA, Network Associates Laboratories