SSL.Constants: Added some more notes about DTLS.
Testsuite [SSL]: Use same DN for the RSA certs as for the others.

The RSA cert is created via a different taste of the
Standards.PKCS.Certificate.build_distinguished_name() than
the DSA and ECDSA certs. Reorder the fields to make sure
that the exact same DN is generated in both cases.

Fixes issue where SSL.Context()->find_cert_issuer() either
didn't find the RSA certs, or didn't found only the RSA certs.
Added HKDF, used by e.g. IKEv2 (IPSec).
Minor clean up.
cert is always set in all callers, and the current code would crash anyay if it wasn't. Remove check.
The auth_level and cert checks are done in verify_certificate_chain, so no need to do them here. The length checks here are bogus and will always fail.
Fixed warning.