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/Password.h

@@ -28,7 +28,7 @@ namespace rfb {
   public:
     PlainPasswd();
     PlainPasswd(char* pwd);
-    PlainPasswd(int len);
+    PlainPasswd(size_t len);
     PlainPasswd(const ObfuscatedPasswd& obfPwd);
     ~PlainPasswd();
     void replaceBuf(char* b);
@@ -37,10 +37,10 @@ namespace rfb {
   class ObfuscatedPasswd : public CharArray {
   public:
     ObfuscatedPasswd();
-    ObfuscatedPasswd(int l);
+    ObfuscatedPasswd(size_t l);
     ObfuscatedPasswd(const PlainPasswd& plainPwd);
     ~ObfuscatedPasswd();
-    int length;
+    size_t length;
   };
 
 }