Replace handshake NULLs with an IDtag

Replace handshake NULLs with an IDtag

Each handshake message contains so called IDtag: XTEA encrypted first 64
bits of transmitted message with client's identity as a key. To
determine if it is handshake message we check all possible client
identities as a key.

Now handshake messages became indistinguishable from the random.

Signed-off-by: Sergey Matveev <stargrave@stargrave.org>