Implement several 'ip' modifiers

* doc/Makefile: Add new examples.
* doc/ip-multicast.md: ditto. 
* doc/ip-proto-ah.md: ditto. 
* doc/ip6-multicast.md: ditto. 
* doc/ip6-proto-ah.md: ditto. 
* src/pf/expand.lua: Add expansion of new modifiers for ip: 'broadcast', 'multicast' and 'proto'.
* src/pf/parse.lua: Add support for parsing of ip modifiers.

this isn't right; "protochain tcp" is valid, i think