PKCS#7: Find the right key in the PKCS#7 key list and verify the signature

Find the appropriate key in the PKCS#7 key list and verify the signature with
it.  There may be several keys in there forming a chain.  Any link in that
chain or the root of that chain may be in our keyrings.

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