PKCS#7: Allow detached data to be supplied for signature checking purposes

It is possible for a PKCS#7 message to have detached data.  However, to verify
the signatures on a PKCS#7 message, we have to be able to digest the data.
Provide a function to supply that data.  An error is given if the PKCS#7
message included embedded data.

This is used in a subsequent patch to supply the data to module signing where
the signature is in the form of a PKCS#7 message with detached data, whereby
the detached data is the module content that is signed.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>