sign-file: Generate CMS message as signature instead of PKCS#7
Make sign-file use the OpenSSL CMS routines to generate a message to be
used as the signature blob instead of the PKCS#7 routines. This allows us
to change how the matching X.509 certificate is selected. With PKCS#7 the
only option is to match on the serial number and issuer fields of an X.509
certificate; with CMS, we also have the option of matching by subjectKeyId
extension. The new behaviour is selected with the "-k" flag.
Without the -k flag specified, the output is pretty much identical to the
PKCS#7 output.
Whilst we're at it, don't include the S/MIME capability list in the message
as it's irrelevant to us.
Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-By: David Woodhouse <David.Woodhouse@intel.com