Rework raw headers storage.

- Use raw headers instead of message headers in message parser
- Store headers of parts
- Use double linked lists to avoid overhead on appending
- Decode raw headers
Remove old mime headers.
Remove stupid lua_message module.
Fix expressions.
Fix filter usage.
Remove useless variables.
Fix regexp module.
Fix DKIM module (+memory leak)
New headers API for lua.
Simplify API.
Fix emails plugin.
Fix forged_recipients plugin.
Fix maillist plugin.
Fix multimap.
Fix configuration.
Fix simple headers API.
Fix maillist.
Merge branch 'headers-rework'