Implement Zlib compression, in both SSH1 and SSH2.

[originally from svn r792]

putty.h

@@ -140,6 +140,7 @@ typedef struct {
     /* SSH options */
     char remote_cmd[512];
     int nopty;
+    int compression;
     int agentfwd;
     enum { CIPHER_3DES, CIPHER_BLOWFISH, CIPHER_DES } cipher;
     char keyfile[FILENAME_MAX];