Implement missing lowering operators

Currently only a small subset of lowering operators are implemented.  This set should be made complete.  Happily it's not too difficult -- you just compile an expression with libpcap, cross-compile to Lua using the pflua bpf compiler, look at the generated code, then make the pflang compiler do something similar.
actually all of the has_proto_{src,dst}_host functions could be factored into one `function has_proto_dir_host(proto, dir, host, mask)` where dir is either the string `src` or `dst`, dunno