Encourage explicit arguments for --decrypt in "show" and "reply"

[Daniel Kahn Gillmor]

The notmuch indexing subcommands ("new", "insert", and "reindex") now
have a --decrypt option that takes an argument (the decryption
policy), since the session-keys patches have landed.

But the viewing subcommands ("show" and "reply") have their
traditional --decrypt option that (as a boolean) need not take an
argument, having --decrypt not present means something different from
either --decrypt=true or --decrypt=false.

This series allows the user to explicitly choose --decrypt=auto for
the viewing subcommands, while allowing people to use the
argument-free form (as an alias for --decrypt=true), but warns the
user to encourage them to switch to using an explicit argument
instead.

This is useful normalizing work for the interface, so it's worthwhile
on its own. It is also necessary preparation in the event that we
decide we want to:

 * set up a notmuch configuration option that changes the default for
   --decrypt for the viewing subcommands
 
 * allow "notmuch show" to actually index encrypted messages upon
   their first encounter (e.g., via a new decryption policy, which
   i'll propose separately)

As always, review and feedback welcome!

       --dkg