crypto: handle PKCS#7 envelopedData in _notmuch_crypto_decrypt
In the two places where _notmuch_crypto_decrypt handles
multipart/encrypted messages (PGP/MIME), we should also handle PKCS#7
envelopedData (S/MIME).
This is insufficient for fully handling S/MIME encrypted data because
_notmuch_crypto_decrypt isn't yet actually invoked for envelopedData
parts, but that will happen in the following changes.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>