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.
Merge pull request #3 from nightlyone/no_netstat_on_linux

Linux specific net statistics