4 implement unblock stop logic for non ring source

[fako1024]

Turns out it's possible to use the PPOLL syscall without a ring buffer (i.e. without a mmap'ed memory area) as well by setting up the same event handling logic and executing the unix.Recvfrom only upon receiving the appropriate event on the socket. This is obviously not "optimal" (in the sense of performance) because unix.Recvfrom was already blocking / waiting for packets, but IMHO this is outweighed by the fact that the event file descriptor is the only way to reliably stop / unblock a blocked capture (needed for graceful stop and rotations in goProbe and maybe similar scenarios, and also to achieve feature parity with the ring buffer source).
Given that the "plain" source is probably not the go-to choice for high-throughput situations anyway I think it's a good bargain. Should also help setting up a shared mock socket in #7 (because both sources now use the same socket / event logic).

Closes #4