Implement several ip modifiers

* Implement lowering 'ip proto <proto>' and 'ip6 proto <proto>'
* Implement lowering 'ip broadcast', '[ip|ip6] multicast' and '[ip|ip6] protochain'.
   * 'ip broadcast' returns an error in libpcap as the netmask is not
      available.
   * 'ip protochain <protocol>' expects an integer value. Fixed error in
      parsing. Left operators unimplemented as the BPF code is rather complex.
      It may be worth to create a ticket for this task.
* Fix bug: expand "ether proto <proto-number>".
Add examples 'ip proto' and 'ether proto <proto-number>'

* Fix: libpcap doesn't admit arithmetic expressions for operators 'less'
and 'greater'.
Merge pull request #58 from Igalia/low-ip-proto

Implement several 'ip' modifiers