PKCS#7: Find intersection between PKCS#7 message and known, trusted keys

Find the intersection between the X.509 certificate chain contained in a PKCS#7
message and a set of keys that we already know and trust.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>