S/MIME patches, v4

[David Bremner]

I took Jamie's advice and simplified the openssl based tests
to ignore the actual output.

I added a test to generate a signed+encrypted message, and verified
that with openssl (in two steps).

I'm a bit stumped why the test introduced in patch 5/5 is failing so
badly. I'd give even odds that I haven't set up gpgsm right (only
imported the public key and not the private one?). Still, it looks to
me like there should be some more diagnostics from notmuch if the
crypto is failing.