Limit access to non-shared mode

A read-only client should not be allowed to kick out other clients.
It will be forced into shared mode, or refused the connection, depending
on the neverShared parameter.
diff --git a/common/rfb/SConnection.h b/common/rfb/SConnection.h
index 0379b17..005a7a8 100644
--- a/common/rfb/SConnection.h
+++ b/common/rfb/SConnection.h
@@ -127,6 +127,7 @@
     static const AccessRights AccessPtrEvents;      // Send pointer events
     static const AccessRights AccessCutText;        // Send/receive clipboard events
     static const AccessRights AccessSetDesktopSize; // Change desktop size
+    static const AccessRights AccessNonShared;      // Exclusive access to the server
     static const AccessRights AccessDefault;        // The default rights, INCLUDING FUTURE ONES
     static const AccessRights AccessNoQuery;        // Connect without local user accepting
     static const AccessRights AccessFull;           // All of the available AND FUTURE rights