Savefile cleanups

* src/pf/savefile.lua: Inline load_packets_from_mmap into load_packets.
  Remove records_mm iterator.  Remove code to write savefiles, as it is
  unused in pflua (pflua-filter does it at a lower level, so as to
  preserve timestamps).

* src/pf.lua: Remove filter_count export; it's not a useful public
  interface (as shown by the fact that nothing but unused code depends
  on it).  Instead inline it into the test where it is used.