X.509: Fix self-signed determination

There's a bug in the code determining whether a certificate is self-signed
or not: if they have neither AKID nor SKID then we just assume that the
cert is self-signed, which may not be true.

Fix this by checking that the raw subject name matches the raw issuer name
and that the public key algorithm for the key and signature are both the
same in addition to requiring that the AKID bits match.

Signed-off-by: David Howells <dhowells@redhat.com>