Add support for parsing /proc/net/netstat

[chengshuyi]

/proc/net/netstat is a very important file and is often used to diagnose problems related to network packet loss. Therefore, many monitoring tools will check this file regularly, so it is valuable to add analysis to this file.

[mwinters0]

FYI I'm WIP on this with a different approach from the previous incomplete PR. This approach will be compatible with both past and future kernels.