smime: Pass PKCS#7 envelopedData to node_decrypt_and_verify

This change means we can support "notmuch show --decrypt=true" for
S/MIME encrypted messages, resolving several outstanding broken tests,
including all the remaining S/MIME protected header examples.

We do not yet handle indexing the cleartext of S/MIME encrypted
messages, though.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>