binpac: Migrate fmt() usage to strfmt()

The former is easy to misuse by accidentally storing the contents of
the temporary string return value and accessing it later.  There's also
potential pitfalls in changing it to return a pointer into a static
buffer, so instead start using strfmt() uniformly across the codebase
and change some methods to use strings instead of char*.

tools/binpac/src/pac_output.h

@@ -19,7 +19,7 @@ protected:
 
 class Output {
 public:
-	Output(const char *filename);
+	Output(string filename);
 	~Output();
 
 	int println(const char* fmt, ...);