crypto: index encrypted parts when indexopts try_decrypt is set.

If we see index options that ask us to decrypt when indexing a
message, and we encounter an encrypted part, we'll try to descend into
it.

If we can decrypt, we add the property index.decryption=success.

If we can't decrypt (or recognize the encrypted type of mail), we add
the property index.decryption=failure.

Note that a single message may have both values of the
"index.decryption" property: "success" and "failure".  For example,
consider a message that includes multiple layers of encryption.  If we
manage to decrypt the outer layer ("index.decryption=success"), but
fail on the inner layer ("index.decryption=failure").

Because of the property name, this will be automatically cleared (and
possibly re-set) during re-indexing.  This means it will subsequently
correspond to the actual semantics of the stored index.