Cleanup postlicyd configuration syntax

[Fruneau]

• provide a cleaner syntax:

on_hookname {
  action1;
  action2;
  next = (filtername|postfix);
}

• the list of action becomes a small scripting language executed in declaration order
• keep compatibility with previous syntax (if on_hookname is followed by a = use previous post-action format)

Might be the key feature of postlicyd 0.9.