Support RFC 4419.

PuTTY now uses the updated version of Diffie-Hellman group exchange,
except for a few old OpenSSH versions which Darren Tucker reports only
support the old version.

FIXME: this needs further work because the Bugs config panel has now
overflowed.

putty.h

@@ -838,6 +838,7 @@ void cleanup_exit(int);
     X(INT, NONE, sshbug_rekey2) \
     X(INT, NONE, sshbug_maxpkt2) \
     X(INT, NONE, sshbug_ignore2) \
+    X(INT, NONE, sshbug_oldgex2) \
     X(INT, NONE, sshbug_winadj) \
     X(INT, NONE, sshbug_chanreq) \
     /*                                                                \