Harmonize MinGW CODESET lookup with MSVC.

Harmonize MinGW CODESET lookup with MSVC.

Historically, MinGW environments lacked some Windows API calls, so we
took a different code path in win32_langinfo().  Somehow, the code
change in commit 35eeea62 (removing setlocale() calls) caused one
particular 001_initdb.pl test to fail on MinGW + ICU builds, because
pg_import_system_collations() found no collations.  It might take a
MinGW user to discover the exact reason.

Updating that function to use the same code as MSVC seems to fix that
test, so lets do that.  (There are plenty more places that test for MSVC
unnecessarily, to be investigated later.)

While here, also rename the helper function win32_langinfo() to
win32_get_codeset(), to explain what it does less confusingly; it's not
really a general langinfo() substitute.

Noticed by triggering the optional MinGW CI task; no build farm animals
failed.

Discussion: https://postgr.es/m/CA%2BhUKGKBWfhXQ3J%2B2Lj5PhKvQnGD%3DsywA0XQcb7boTCf%3DerVLg%40mail.gmail.com