File - locking

- Made locking an integral part of the File object, so that a client
  may now call File::lock multiple times with no effect beyond the first
  call.  This simplifies situations like the 'cal' command that must scan
  for recurrence changes, modify tasks, then optionally run a second
  report, all without locking issues.

src/File.h

@@ -82,6 +82,7 @@ public:
 private:
   FILE* fh;
   int   h;
+  bool  locked;
 };
 
 #endif