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/parameters.cxx b/vncviewer/parameters.cxx
index 9a9b9fd..1151a7c 100644
--- a/vncviewer/parameters.cxx
+++ b/vncviewer/parameters.cxx
@@ -120,6 +120,9 @@
 BoolParameter acceptClipboard("AcceptClipboard",
                               "Accept clipboard changes from the server",
                               true);
+BoolParameter setPrimary("SetPrimary",
+                         "Set the primary selection as well as the "
+                         "clipboard selection", true);
 BoolParameter sendClipboard("SendClipboard",
                             "Send clipboard changes to the server", true);
 #if !defined(WIN32) && !defined(__APPLE__)