[feature] Remove expensive net.Interface lookups upon link initialization

[fako1024]

This should alleviate the performance issues encountered on hosts with lots of interfaces. The existing call via net.Interface causes a full routing table lookup upon each interface initialization (which is of course especially large on hosts with many interfaces, causing a mean amplification effect). Since we do not require the information therein to setup the ring buffer I've simply replaced the existing implementation with my own lookup based on /sys/class/net (which I was using to determine the link type before already) so that only the information for the interface in question is accessed an no routing table information is fetched.
This of course means that we'd need a different implementation if we ever support more OSes, but I'm sure that would have been the case anyway (because we can't use AF_PACKET anywhere else), plus we could always fall back to net.Interface in such cases.

Closes #50

[fako1024]

@els0r PTAL when you have a moment. Once this is merged I'll bump the version in goProbe and together with the changes done in the recent rotation optimization we can take another shot at profiling a 300+ interfaces host for comparison if you like... 😉

[els0r]

@els0r PTAL when you have a moment. Once this is merged I'll bump the version in goProbe and together with the changes done in the recent rotation optimization we can take another shot at profiling a 300+ interfaces host for comparison if you like... 😉

I sure can :)