strftime_s

Since strftime(false, "%s") doesn't work on Windows implement a qc version of it (strftime_s()).

Note: as reported by bones_was_here in a comment of this MR, in his system strftime(false, "%s") returns a slightly wrong number (-1 hour difference), whereas strftime_s() returns the correct number. Anyway a difference of 1 hour doesn't matter for the sake of having an unique progressive number for numbering screenshot filenames and for making a match ID.

It fixes #2588 (closed) "autoscreenshot fails to append a number to screenshot filenames under Windows"