Add support for the SUPDUP protocol.

Based on work by Josh Dersch, with permission.

be_nossh.c

@@ -13,6 +13,7 @@ const char *const appname = "PuTTYtel";
 const struct BackendVtable *const backends[] = {
     &telnet_backend,
     &rlogin_backend,
+    &supdup_backend,
     &raw_backend,
     NULL
 };