Require all SMsgWriter caller to check capabilities

Make the API consisitent by requiring the caller to check what the client
supports before calling any of the write* functions. This avoids the
confusion that the functions might not always do anything.
diff --git a/common/rfb/SMsgWriter.h b/common/rfb/SMsgWriter.h
index 19b013f..80f6de9 100644
--- a/common/rfb/SMsgWriter.h
+++ b/common/rfb/SMsgWriter.h
@@ -69,17 +69,17 @@
     // write the relevant pseudo-rectangle as part of the next update.
     void writeDesktopSize(rdr::U16 reason, rdr::U16 result=0);
 
-    bool writeSetDesktopName();
+    void writeSetDesktopName();
 
     // Like setDesktopSize, we can't just write out a cursor message
     // immediately. 
     void writeCursor();
 
     // Same for LED state message
-    bool writeLEDState();
+    void writeLEDState();
 
     // And QEMU keyboard event handshake
-    bool writeQEMUKeyEvent();
+    void writeQEMUKeyEvent();
 
     // needFakeUpdate() returns true when an immediate update is needed in
     // order to flush out pseudo-rectangles to the client.