Rename PRI__s64 to PRI__d64

As PRI__*64 macros mirror PRI*64 macros from inttypes.h, follow the
principle of least astonishment and name these macros the same way.

* defs.h (PRI__s64): Rename to PRI__d64.
diff --git a/defs.h b/defs.h
index ac59349..0b5f9f9 100644
--- a/defs.h
+++ b/defs.h
@@ -787,6 +787,6 @@
 # define PRI__64 "ll"
 #endif
 
-#define PRI__s64 PRI__64"d"
+#define PRI__d64 PRI__64"d"
 #define PRI__u64 PRI__64"u"
 #define PRI__x64 PRI__64"x"