Use the spot::formater class in ltlcheck to format output commands.

* src/misc/formater.hh (printable_value): Add more accessors.
* src/bin/ltlcheck.cc: Use the formater class to factor the %-expansion
loop, and reuse filenames (such as %F, %S, and %L) between executions
on the same formula.

src/misc/formater.hh

@@ -45,11 +45,26 @@ namespace spot
   protected:
     T val_;
   public:
-    operator T() const
+    const T& val() const
     {
       return val_;
     }
 
+    T& val()
+    {
+      return val_;
+    }
+
+    operator const T&() const
+    {
+      return val();
+    }
+
+    operator T&()
+    {
+      return val();
+    }
+
     printable_value&
     operator=(const T& new_val)
     {