Refactor local time getter functions (#12221)

Refactor local time getter functions (#12221)

This commit introduces mt_localtime() in src/gettime.h, a wrapper
around the OS-specific thread-safe versions of localtime()
(resp. localtime_s on Windows and localtime_r in other systems).

Per the Open Group recommendation,
«portable applications should call tzset() explicitly before using
ctime_r() or localtime_r() because setting timezone information is
optional for those functions», so we also do a one-shot
call of tzset() (_tzset() on Windows to avoid warning C4996).

The function is used to replace the localtime() calls in
getTimestamp() and makeScreenshot().

(The only reminaing call to localtime() in the tree now is the one in
the local copy of the Lua source code.)