Currently there is no easy way to specify net.isr.maxthreads = all cpus. We need
to specify exact number of cpus in loader.conf which get annoying when you have
mix of machines which don't have equal number of total cpus. I propose "-1" as
that value. When loader.conf has net.isr.maxthreads = -1, netisr will use all
available cpus.

In collaboration with: davide
Reviewed by: gnn
Differential Revision: https://reviews.freebsd.org/D2318
MFC after: 2 weeks
Sponsored by: Limelight Networks