onionmasq: improve CLI usability, replace netlink with fwmarks

• The onionmasq Linux testing CLI gets a bit of love, using clap to make it behave more like a proper command-line app with argument parsing.
• The default log level is trimmed down massively to basically say nothing, and you can enable per-connection logging (-v) or debug logging (--debug) if desired.
• Specifying an exit country code is now supported.
• Instead of using netlink to add a passthrough route (which is kind of brittle, and didn't support IPv6), instead attempt to tag outgoing sockets with a custom fwmark.
  • This can be used to do policy routing, e.g. ip rule add not fwmark 0xc185 table xyz.
  • A future version of the CLI will attempt to do this automagically!