Make the SSH2 traffic analysis defence robust in the face of Zlib

compression. This involves introducing an option to disable Zlib
compression (that is, continue to work within the Zlib format but
output an uncompressed block) for the duration of a single packet.

[originally from svn r982]

ssh.h

@@ -159,6 +159,7 @@ struct ssh_compress {
     void (*decompress_init)(void);
     int (*decompress)(unsigned char *block, int len,
 		      unsigned char **outblock, int *outlen);
+    int (*disable_compression)(void);
 };
 
 struct ssh2_userkey {