Nettle: Fix compilation on non-Linux.

Use the MINIMUM() macro which always exists, instead of the
MIN() macro which exists on Linux.
SSL.context: Support any cert when there's no SNI.

If the client doesn't send an SNI, we can't filter on it...
SSL.handshake: Fixed remaining fmt_constant() call.
Updated comment.
Standards.PKCS.Identifiers: Added on_dnsSRV_id from RFC 4985.

This is the Subject Alternative Name for Expression of Service Name
X.509 extension.