Add BPF disassembler, fix BPF compilation bug

* src/pf/bpf.lua (disassemble): New function.
  (compile_lua): Print BPF disassembly in verbose mode.  Fix BPF_TAX
  instruction (!).
Merge pull request #37 from Igalia/disassemble-bpf

Add BPF disassembler, fix BPF compilation bug