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 f1da075d9a), 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.munro@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKGJqVe0%2BPv9dvC9dSums_PXxGo9SWcxYAMBguWJUGbWz-A%40mail.gmail.com

configure.ac

@@ -1764,6 +1764,7 @@ AC_CHECK_FUNCS(m4_normalize([
 	getpeerucred
 	inet_pton
 	kqueue
+	localeconv_l
 	mbstowcs_l
 	memset_s
 	posix_fallocate