Add neighbor state

[eugercek]

Implements part of #535

Some things we can talk:

• We can omit state="stale" like labels and just give the state value to Gauge (IMO current impl is simpler) although I saw below in CONTRIBUTING.md I didn't see many magic numbers in the metrics and labels thus added human readable names for them.

For example a proc file contains the magic number 42 as some identifier, the Node Exporter should expose it as it is and not keep a mapping in code to make this human readable.

• For code quality I can split Update function to make it more readable (again IMO it's simple enough to do it in one function)
• Can remove neighborState struct and just use string slice, but this is easier to follow IMO
• This change may create many metrics, to make a estimation Default values of arp cache size are: min 128 and max 1024 therefore we may except up to 1024*count(net_int) as max.

@SuperQ it's ready for your review, when you have time 🙏🏻