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.

Right, I got a syntax error when I tried it because I needed to escape the protocol keyword:

"ip protochain \\tcp"

So "ip protochain" admits a protocol keyword or an integer, and the same is valid for "ip proto", "ip6 proto" and "ether proto".