[crypto] Ensure that a closed channel is left with unusable ciphers

We currently protect against a consumer that erroneously uses values
generated from the ephemeral master secret after closing a previously
opened channel, by deliberately replacing the ephemeral master secret
rather than zeroing it when the channel is closed.

Extend this concept to protect against a consumer that erroneously
uses the ciphers after closing a previously opened channel (or that
erroneously uses the ciphers from a channel that failed to open
successfully), by using the dead ciphers by default and by enabling
the plaintext (null) ciphers when and only when the channel has been
successfully opened.

Signed-off-by: Michael Brown <mcb30@ipxe.org>