crypto: add --decrypt=nostash to avoid stashing session keys

crypto: add --decrypt=nostash to avoid stashing session keys

Here's the configuration choice for people who want a cleartext index,
but don't want stashed session keys.

Interestingly, this "nostash" decryption policy is actually the same
policy that should be used by "notmuch show" and "notmuch reply",
since they never modify the index or database when they are invoked
with --decrypt.

We take advantage of this parallel to tune the behavior of those
programs so that we're not requesting session keys from GnuPG during
"show" and "reply" that we would then otherwise just throw away.