cli/new, insert, reindex: change index.decrypt to "auto" by default

The new "auto" decryption policy is not only good for "notmuch show"
and "notmuch reindex".  It's also useful for indexing messages --
there's no good reason to not try to go ahead and index the cleartext
of a message that we have a stashed session key for.

This change updates the defaults and tunes the test suite to make sure
that they have taken effect.