Fixed encrypted mails

The structure in which we created encrypted mails was not compatible
with what the enigmail plugin expects (because it's apparently a limited
implementation that makes some assumptions), for roundcube and
thunderbird. This is now fixed to generate mail in-line with what the
other clients do:

• The toplevel part is always the signed/encrypted part. Below we then

have e.g. a multipart/alternative if we have attachments.

• The attached key is added as a regular attachment that ends up inside

the encrypted/signed message.