Split BPF compiler out of pf.lua

* src/pf/bpf.lua: New file, split out of pf.lua.
* src/pf.lua: Adapt.
* src/pf/libpcap.lua: Adapt, and take verbose parameter from presence of
  PF_VERBOSE environment var.
* src/Makefile: Update.

* src/pf_test.lua: Update to do its own elapsed time checking.