'port' and 'portrange' can be prefixed by 'src' or 'dst'.

- Implement lowering '[src|dst] port <port-number>'.
- Implement lowering '[src|dst] portrange <portrange>'.
- Add examples for both cases.
Update old doc examples

- Statement "return false" is now wrapped as "do return false end".
Merge pull request #52 from Igalia/low-port-number

Operators 'port' and 'portrange' can be prefixed by 'src' or 'dst'