Investigate support for the null cipher in the network protocol

[alexrp]

Specifically, need to check whether sending 0 in the key exchange works as expected. Also check whether the client actually expects to exchange keys at all when using the null cipher.

[alexrp]

Further RE indicates that the client does not expect to exchange keys when using NullCipher. The socket is considered ready immediately upon receiving the CIPHER_KIND_NONE from the server.

enum CipherKind : uint32_t
{
    CIPHER_KIND_NONE = 0x0,
    CIPHER_KIND_PIKE = 0x1,
};

Should be easy enough to support.

[alexrp]

Implementation and documentation covered by #15 and #1, respectively.