Handle PKCS#7 signedData (S/MIME single-part clearsigned)

[William Casarin]

Hey Daniel,

Daniel Kahn Gillmor <dkg at fifthhorseman.net> writes:
> Several of the patches should be simple to read and uncontroversial (I
> hope!), and I welcome/encourage merging of those patches to reduce the
> length of the remaining series.

I did a quick skim and it looks ok, although I'm not super familiar with
S/MIME. Any steps toward S/MIME support gets a Concept ACK from me!

Cheers,
Will