make a first pass at providing kstats from the hardware counters.

make a first pass at providing kstats from the hardware counters.

mvneta counters are read to clear, so we accumulate them over time.
they are also mostly 32bit, except for 2 of them, so this handles
all the 32bit counters in a loop and deals with the 64bit ones as
exceptions.

i can see rx drops now, but not the rx rx counters yet. i'll figure
it out.

this is mostly inspired by em(4).