pgsql: Use thread-safe strftime_l() instead of strftime().

Use thread-safe strftime_l() instead of strftime().

This removes some setlocale() calls and a lot of commentary about how
dangerous that is. strftime_l() is from POSIX 2008, and on Windows we
use _wcsftime_l().

While here, adjust error message for strftime_l() failure: it does not
in practice set errno (even though POSIX says it could), so no %m.

Author: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/CA%2BhUKGJqVe0%2BPv9dvC9dSums_PXxGo9SWcxYAMBguWJUGbWz-A%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/890fc826c91f153ae8c084d3a008f91798495b13

Modified Files
--------------
src/backend/main/main.c | 5 +-
src/backend/utils/adt/pg_locale.c | 110 ++++++++-------------------------
src/backend/utils/adt/pg_locale_libc.c | 3 +-
src/include/utils/pg_locale.h | 1 +
4 files changed, 29 insertions(+), 90 deletions(-)