crypto: record whether an actual decryption attempt happened

crypto: record whether an actual decryption attempt happened

In our consolidation of _notmuch_crypto_decrypt, the callers lost
track a little bit of whether any actual decryption was attempted.

Now that we have the more-subtle "auto" policy, it's possible that
_notmuch_crypto_decrypt could be called without having any actual
decryption take place.

This change lets the callers be a little bit smarter about whether or
not any decryption was actually attempted.