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/vncviewer.man b/vncviewer/vncviewer.man
index 5dd3b45..4b92b61 100644
--- a/vncviewer/vncviewer.man
+++ b/vncviewer/vncviewer.man
@@ -153,6 +153,11 @@
 Accept clipboard changes from the server. Default is on.
 .
 .TP
+.B \-SetPrimary
+Set the primary selection as well as the clipboard selection.
+Default is on.
+.
+.TP
 .B \-SendClipboard
 Send clipboard changes to the server. Default is on.
 .