MH should deal with attachments that are separately PGP'd

[massar]

Validity needs to be confirmed:

if an attachment is separately encrypted (so, not in RFC 2015, but still good PGP once it's de-fanged from base64), and some earlier attachment was PGP encrypted; or if all attachments are either separately signed, separately encrypted, or separately signed+encrypted, then consider promoting the whole mess into one RFC 2015 blob for re-encryption. right now separately encrypted attachments (from thunderbird) are stripped.