Implement several ethernet modifiers

* src/pf/expand.lua: add expansion of ethernet modifiers 'broadcast', 'multicast', and 'proto <proto>'.
* src/pf/parse.lua: add support for parsing 'broadcast', 'multicast', and 'proto <proto>'. Ethernet protocols also work as keywords. Modifiers 'decnet' and 'iso' can have modifiers or work as single words.
* doc/ether-broadcast.md: Example of 'ethernet broadcast'.
* doc/ether-multicast.md: Example of 'ethernet multicast'.
* doc/ether-proto-decnet.md: Example of 'decnet'.

whoops, i guess "ether proto ip" is exactly the same as "ip"!  as you note below.  please disregard the previous comment.  this looks great :)