'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'
Implement lowering 'ether broadcast'

According to the BPF syntax document, ether is optional, so 'broadcast'
is the same as 'ether broadcast'.
Implement lowering 'ether multicast'

According to the BPF syntax document, ether is optional, so 'multicast'
is the same as 'ether multicast'.
Implement lowering 'ether proto <proto>'

protos: ip, ip6, arp, rarp, atalk, aarp, decnet, sca, lat, mopdl, moprc, iso,
 stp, ipx, or netbeui. These identifiers are also keywords.
Add doc examples for 'ether multicast', 'ether broadcast' and 'ether proto <proto>'.