add and sub with runtime routines

* src/pf.lua: Implement add and sub with runtime functions, as they need
  to do modular arithmetic, not clamping arithmetic.  Slight
  optimizations to BPF_IND addressing mode.
Compile filter in optimized mode; minor cleanups and optimizations

* src/pf.lua: Rename MAX_UINT32 to MAX_UINT32_PLUS_1.  Try to do
  strength reduction on MUL and DIV instructions, and prohibit DIV on
  non-constant RHS to prevent division-by-zero; pcap at least does not
  seem to generate DIV anyway.  Tell pcap to optimize the filter.
Minor cleanups

* src/pf.lua: Minor cleanups.
Fix bug in JMP instructions

* src/pf.lua (bpf_program): Fix bug in JMP instructions.