Implement symbolic constants.

- Symbolic constants in arithmetic expressions. Add support for
constansts: 'protocol header field offsets', 'icmp type field values'
and 'tcp flag field values'.

- Symbolic constants in logical operations. Add support for constants
names for port numbers. Valid in 'port <port>' and 'portrange <port-port>'
contexts.

This patch solves issue #24.
Merge pull request #24 from Igalia/issue-24-implement-symbolic-constants

Implement symbolic constants