[Packet] Sizes should be unsigned

A size can't possibly be negative. Use size_t as a convenient unsigned
int.
[ProbeRequestFilter] Make the ProbeRequest fields public

We're using ProbeRequest as a simple struct for the moment, so the
fields should be publicly accessible.

We'll probably want to change that later, but for now this will suffice.
[ProbeRequestFilter] Assert that we have enough data

Don't try to parse the radiotap header if we don't have enough data for
it.
That's never supposed to happen, so an assert is entirely appropriate.
[ProbeRequestFilter] Parse the IEEE802.11 frame header

Ignore everything that isn't a probe request and save the MAC address of
the node making the probe request.
Emit probeRequest() signals.
[ProbeRequestFilter] Cleanup

 - Remove it_ prefix from the radiotap header definition. They're pretty
   useless. It should be obvious from the variable names what type of
   struct we're dealing with.

 - Rename the radiotap header to use the _hdr suffix, just like the
   ieee80211_hdr.