Provide extended net_io_counters() metrics

[giampaolo]

The same way we added extended platform specific disk IO metrics on Linux and FreeBSD (busy_time and merged r/w times, see #523 and #756) we should do the same for net_io_counters(). There's a different metrics we are currently ignoring and not returning just for the sake of cross-platform conformity.

Existent metrics

• bytes_sent
• bytes_recv
• packets_sent
• packets_recv
• errin
• errout
• dropin
• dropout

Linux

ref: http://www.onlamp.com/pub/a/linux/2000/11/16/LinuxAdmin.html

• fifoin/out: num of FIFO buffer errs
• framein: num of frame errs
• compressedin: num of compressed incoming pkts
• multicastin: num of multicast incoming frames
• collisionsout: num of outgoing collision errs
• carrierout: num of outgoing carrier losses
• compressedout: num of outgoing compressed pkts

Windows

ref: https://msdn.microsoft.com/en-us/library/windows/desktop/aa366836(v=vs.85).aspx

• unicastin/out
• nonunicastin/out
• unknownprotoin: the number of incoming packets that were discarded because the protocol was unknown

BSD

ref: ifdata struct in net/if.h

• collisions: (both in and out)
• multicastin/out

Linux patch

        # in
        (bytes_recv,
         packets_recv,
         errin,
         dropin,
         fifoin,  # ---
         framein,
         compressedin,
         multicastin,
         # out
         bytes_sent,
         packets_sent,
         errout,
         dropout,
         fifoout,  # ---
         collisionsout,
         carrierout,
         compressedout) = map(int, fields)