Fix bugs around wrong argument to gettimeofday, pcap_offline_filter

* src/pf.lua: Create a pcap_pkthdr instead of a pcap_record.  They're
  different on 64-bit systems...

* src/pf/libpcap.lua: Update pcap_offline_filter prototype and selftest
  message.

* src/pf/savefile.lua: Remove struct pcap_record_extra, as it's not
  used.

* src/pf/types.lua: Add pcap_pkthdr.

* src/pf/utils.lua: Fix suseconds_t type (!).
Fix compile_filter for pcap_offline_filter

* src/pf.lua (compile_filter): For pcap_offline_filter, just allocate
  one header and mutate its values for each packet.  Seems to prevent
  JIT from seeing the header as dead after allocation.
Add comment on now() and a test

* src/pf/utils.lua: Add a comment about now's epoch, and a test.