cli: include wrapped part of PKCS#7 SignedData in the MIME tree

Unwrap a PKCS#7 SignedData part unconditionally when the cli is
traversing the MIME tree, and return it as a "child" of what would
otherwise be a leaf in the tree.

Unfortunately, this also breaks the JSON output.  We will fix that
next.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>