Move everything in util.h to zeek::util namespace.

This commit includes renaming a number of methods prefixed with bro_ to be prefixed with zeek_.

src/threading/MsgThread.h

@@ -386,7 +386,7 @@ protected:
 	 * mainly for debugging purposes.
 	 */
 	explicit Message(const char* arg_name)
-		{ name = copy_string(arg_name); }
+		{ name = zeek::util::copy_string(arg_name); }
 
 private:
 	const char* name;