Add option to set primary selection for cut text

Previously the incoming clipboard was unconditionally set to both
the PRIMARY and CLIPBOARD selection. This isn't always what the
user want, so make it configurable.
diff --git a/vncviewer/CConn.cxx b/vncviewer/CConn.cxx
index 262dd2c..140b1d6 100644
--- a/vncviewer/CConn.cxx
+++ b/vncviewer/CConn.cxx
@@ -439,7 +439,8 @@
 
   // RFB doesn't have separate selection and clipboard concepts, so we
   // dump the data into both variants.
-  Fl::copy(buffer, ret, 0);
+  if (setPrimary)
+    Fl::copy(buffer, ret, 0);
   Fl::copy(buffer, ret, 1);
 
   delete [] buffer;