Use size_t for lengths in stream objects

Provides safety against them accidentally becoming negative because
of bugs in the calculations.

Also does the same to CharArray and friends as they were strongly
connection to the stream objects.

common/rfb/util.h

@@ -49,8 +49,8 @@ namespace rfb {
   public:
     CharArray() : buf(0) {}
     CharArray(char* str) : buf(str) {} // note: assumes ownership
-    CharArray(int len) {
-      buf = new char[len];
+    CharArray(size_t len) {
+      buf = new char[len]();
     }
     ~CharArray() {
       delete [] buf;