Linux specific net statistics

```netstat -s``` has 32bit overflow issues on some systems.

Since Linux already provides easy to parse SNMP statistics via /proc
just provide a special implementation and build net.go along with their
tests only for non-linux systems.

As a side effect I also observed reduced memory footprint
and increased execution speed.

@tobstarr please take a look. I observed negative values with ```netstat -s```, 
so I traded a bit more code for better performance and correctness.