[tz] strftime %s

[Dag-Erling Smørgrav]

Currently, strftime() implements %s by calling mktime() and then
printing the result.  This is fine when the struct tm passed to
strftime() came from localtime() but not when it didn't.  A better
solution would be to call timegm() and then manually adjust the result.
Of course that's only possible in the TM_GMTOFF case but that's still
better than nothing.

The first attachment is a program which demonstrates the issue.  It
should print the same value for %s in both cases but doesn't:

% cc -Wall -Wextra -o strftime_s strftime_s.c
% ./strftime_s
local	1704903586 2024-01-10 17:19:46 CET
gm	1704899986 2024-01-10 16:19:46 UTC

The second attachment is my proposed fix.

DES
--
Dag-Erling Smørgrav - des at des.no

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