net-test: packetdrill encap support: refactor packet receive loops into one loop

Factor out the packet sniffing loop that's common to local and remote
mode, since it is nearly identical in both cases. This is in
preparation for fixing a bug that is found in both loops, so we can
make the fix in just one place.


Change-Id: Ic9a9b84b66a9f812866b1e29cd7c897f7c1d40cf