Fix ip proto

* src/pf/expand.lua: 
   - Fix operators 'ip proto', 'ip6 proto' and 'iso proto'. These operators can accept a string or a number as parameter. The number case was omitted.
   - Fix operator 'proto', equivalent to has_ipv4_proto or has_ipv6_proto.
* doc/Makefile: Add examples for operators fixed.
* doc/ip-proto-47.md
* doc/ip6-proto-47.md
* doc/iso-proto-47.md
* doc/proto-47.md

this control flow is getting a little complicated; can we:
local proto = expr[2]
if type(proto) == 'string' then proto = assert(ip_protos[expr[2]], "Invalid IP protocol") end
return have_ipv5_protocol(proto)