Don't expose Windows' mbstowcs_l() and wcstombs_l().

Don't expose Windows' mbstowcs_l() and wcstombs_l().

Windows has similar functions with leading underscores.  Previously, we
provided the rename via a macro in win32_port.h.  In fact its functions
are not always good replacements for the Unix functions, since they
can't deal with UTF-8.  They are only currently used by pg_locale.c,
which is careful to redirect to other Windows routines for UTF-8.  Given
that portability hazard, it seem unlikely to be a good idea to encourage
any other code to think of these functions as being available outside
pg_locale.c.  Any code that thinks it wants these functions probably
wants our wchar2char() or char2wchar() routines instead, or it won't
actually work on Windows in UTF-8 databases.

Furthermore, some major libc implementations including glibc don't have
them (they only have the standard variants without _l), so external code
is very unlikely to require them to exist.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKG%2Bt_CHPzEoPnKyARJBJgE9-GxNajJo6ZuSfRK_KWFO%2B6w%40mail.gmail.com