Fix cannot locate function 'compile_filter'.

When generating BPF asm, the path to function 'compile_filter' is broken. It's necesarry to import 'pf'.

Additional improvements:

   * Removed unused code in 'pc_pcap_asm.lua'.
   * Renamed script 'pc_pcap_asm' to 'bpf-to-asm'.
   * Added usage information to 'bpf-to-asm'.
   * Moved script to dir 'utils/'.