New wrapper macro for printf("%zu"), for old VS compat.

A user reports that Visual Studio 2013 and earlier have printf
implementations in their C library that don't support the 'z' modifier
to indicate that an integer argument is size_t. The 'I' modifier
apparently works in place of it.

To avoid littering ifdefs everywhere, I've invented my own inttypes.h
style macros to wrap size_t formatting directives, which are defined
to %zu and %zx normally, or %Iu and %Ix in old-VS mode. Those are in
defs.h, and they're used everywhere that a %z might otherwise get into
the Windows build.

(cherry picked from commit 82a7e8c4ac)

windows/winpgnt.c

@@ -884,7 +884,7 @@ static char *answer_filemapping_message(const char *mapname)
         mapsize = mbi.RegionSize;
     }
 #ifdef DEBUG_IPC
-    debug("region size = %zd\n", mapsize);
+    debug("region size = %"SIZEu"\n", mapsize);
 #endif
     if (mapsize < 5) {
         err = dupstr("mapping smaller than smallest possible request");