[notmuch] Show mode enhancements

[camalot at picnicpark.org]

I started using notmuch a few days ago and was astounded at how much
more efficiently I could process email with it. For my usage I needed
a bit more flexibility in replying/forwarding messages and the ability
to conveniently save attachments.  The set of patches that follow
contain these two enhancements, a few minor fixes, and the addition of
a binding of 's' in show mode to notmuch-search.

I'd be happy to address anything neded to get these changes into shape
to be included in the main distribution.  Thanks for all your work
putting together such a promising email client.

       --- Keith