Clean up internal clipboard handling

We now filter incoming data, which means we can start assuming the
clipboard data is always null terminated. This allows us to clean
up a lot of the internal handling.
diff --git a/common/rfb/SMsgWriter.h b/common/rfb/SMsgWriter.h
index 4f4c9cc..8cf2ae7 100644
--- a/common/rfb/SMsgWriter.h
+++ b/common/rfb/SMsgWriter.h
@@ -56,7 +56,7 @@
 
     // writeBell() and writeServerCutText() do the obvious thing.
     void writeBell();
-    void writeServerCutText(const char* str, int len);
+    void writeServerCutText(const char* str);
 
     // writeFence() sends a new fence request or response to the client.
     void writeFence(rdr::U32 flags, unsigned len, const char data[]);