PKCS#7: Check content type and versions

We only support PKCS#7 signed-data [RFC2315 sec 9] content at the top level,
so reject anything else.  Further, check that the version numbers in
SignedData and SignerInfo are 1 in both cases.

Note that we don't restrict the inner content type.  In the PKCS#7 code we
don't parse the data attached there, but merely verify the signature over
it.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-By: David Woodhouse <David.Woodhouse@intel.com>