Only public keys, with certificates signed by an existing
'trusted' key on the system trusted keyring, should be added
to a trusted keyring.  This patch adds support for verifying
a certificate's signature.

This is derived from David Howells pkcs7_request_asymmetric_key() patch.
Signed-off-by: NMimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>