refactor timefn

The timer storage type is no longer dependent on OS.
This will make it possible to re-enable posix precise timers
since the timer storage type will no longer be sensible to #include order.
See #3168 for details of pbs of previous interface.

Suggestion by @terrelln

programs/fileio_common.h

@@ -122,4 +122,4 @@ extern UTIL_time_t g_displayClock;
 #if defined (__cplusplus)
 }
 #endif
-#endif //ZSTD_FILEIO_COMMON_H
+#endif /* ZSTD_FILEIO_COMMON_H */