Naive conversion to use BPF for filters.

This just uses libpcap, when it should include the JIT instead, and
probably brilter_pass should be hand-inlined.  And it'd be nice to be
able to pull the filters from a file.  And maybe even support a more
complex set of matching criteria and decisionmaking.  Using Lua to
generate the BPF might make sense, along with additional control
structures.
Rewrite previous filters in BPF.

Experimental and untested, definitely rough around the edges and with
minor changes in logic, particularly around allowing multicast.
Add XXX about pcap-filter's shortcomings.