smtp: create raw-extraction feature

smtp: create raw-extraction feature

Add a raw-extraction option for smtp. When enabled, this feature will
store the raw e-mail inside a file, including headers, e-mail content,
attachments (base64 encoded). This content is stored in a normal File *,
allowing for normal file detection.
It'd also allow for all-emails extraction if a rule has
detect-filename:"rawmsg" matcher (and filestore).
Note that this feature is in contrast with decode-mime.

This feature is disabled by default, and will be disabled automatically
if decode-mime is enabled.