pgsql: Fix incorrect message-printing in win32security.c.

Fix incorrect message-printing in win32security.c.

log_error() would probably fail completely if used, and would
certainly print garbage for anything that needed to be interpolated
into the message, because it was failing to use the correct printing
subroutine for a va_list argument.

This bug likely went undetected because the error cases this code
is used for are rarely exercised - they only occur when Windows
security API calls fail catastrophically (out of memory, security
subsystem corruption, etc).

The FRONTEND variant can be fixed just by calling vfprintf()
instead of fprintf().  However, there was no va_list variant
of write_stderr(), so create one by refactoring that function.
Following the usual naming convention for such things, call
it vwrite_stderr().

Author: Bryan Green <dbryan.green@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAF+pBj8goe4fRmZ0V3Cs6eyWzYLvK+HvFLYEYWG=TzaM+tWPnw@mail.gmail.com
Backpatch-through: 13

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/03bf7a12c5a44ced377352c8f9bf6e9f4b863885

Modified Files
--------------
src/backend/utils/error/elog.c | 16 +++++++++++++---
src/include/utils/elog.h       |  1 +
src/port/win32security.c       |  4 ++--
3 files changed, 16 insertions(+), 5 deletions(-)