Implement lowering 'proto' and 'protochain'

* 'proto <protocol>' is the same as 'ip proto <protocol>'.
* 'protochain <protocol>' is the same as 'ip protochain <protocol>'.
Add example 'proto sctp'
Merge pull request #64 from Igalia/low-proto

Implement lowering 'proto' and 'protochain'