Rewrite main firewall config file

Main ferm configuration file has been rewritten to remove some old
unused code and add new functions to base firewall:

- a comment indicating that rules are generated by ferm is removed;
- ferm now exports 3 variables indicating what "domains" (IPv4, IPv6)
  are active, you can use them in your own ferm configuration;
- new SYN flood protection code based on 'iptables hashlimit' module has
  been added, with limits tweakable using Ansible variables. ICMP is also
  protected by a similar set of rules;