libssl: rework cert signature security level

libssl: rework cert signature security level

This switches to using the X509_get_signature_info() API instead of hand
rolling a part of it. This is slightly tangly since the security level API
is strange. In particular, some failures are passed to the security level
callback so that applications can override them.

This makes the security level API handle RSA-PSS and EdDSA certificates
correctly and the handshake with such can progress a bit further. Of note,
we check that the certs are actually suitable for use in TLS per RFC 8446
contrary to what OpenSSL does.

ok beck jsing