pgsql: Provide thread-safe pg_localeconv_r().

Provide thread-safe pg_localeconv_r().

This involves four different implementation strategies:

1. For Windows, we now require _configthreadlocale() to be available
and work (commit f1da075d9a0), and the documentation says that the
object returned by localeconv() is in thread-local memory.

2. For glibc, we translate to nl_langinfo_l() calls, because it
offers the same information that way as an extension, and that API is
thread-safe.

3. For macOS/*BSD, use localeconv_l(), which is thread-safe.

4. For everything else, use uselocale() to set the locale for the
thread, and use a big ugly lock to defend against the returned object
being concurrently clobbered. In practice this currently means only
Solaris.

The new call is used in pg_locale.c, replacing calls to setlocale() and
localeconv().

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/b98be8a2a2a6196dc24631dfe4d8785b86800d23

Modified Files
--------------
configure | 2 +-
configure.ac | 1 +
meson.build | 1 +
src/backend/utils/adt/pg_locale.c | 128 +++----------
src/include/pg_config.h.in | 3 +
src/include/port.h | 6 +
src/port/Makefile | 1 +
src/port/meson.build | 1 +
src/port/pg_localeconv_r.c | 368 ++++++++++++++++++++++++++++++++++++++
9 files changed, 403 insertions(+), 108 deletions(-)