Filter functions always return booleans

* src/pf.lua (compile_pcap_filter): Return a closure that checks that
  the numeric result is not zero.  Adapt test suite.

* src/pf/libpcap.lua: Rename pcap_offline_filter to offline_filter.
Change pf.lua to export "compile_filter" function

* src/pf.lua: Rename compile_pcap_filter to compile_filter, and take a
  table as options.  Adapt tests.

* src/pf/savefile.lua: Use types.pcap_record.

* src/pf/types.lua (pcap_record): New export.

* src/pf_pcap_asm.lua:
* src/bpf_pflua_ts.lua: Update for new compile_filter syntax.