onion-tunnel: Hide `pcap` behind a feature flag

This commit hides the pcap related functionality behind a conditional compilation flag because it is a dangerous feature for productive systems, but crucial for debugging.

In the onionmasq-apple and onionmasq-mobile crates it makes use of these features, although it remains discussable to whether hide it in those crates behind a similarly named flag as well.

Fixes #131 (closed)