pgsql: Replace pgwin32_is_junction() with lstat().

Replace pgwin32_is_junction() with lstat().

Now that lstat() reports junction points with S_IFLNK/S_ISLINK(), and
unlink() can unlink them, there is no need for conditional code for
Windows in a few places.  That was expressed by testing for WIN32 or
S_ISLNK, which we can now constant-fold.

The coding around pgwin32_is_junction() was a bit suspect anyway, as we
never checked for errors, and we also know that errors can be spuriously
reported because of transient sharing violations on this OS.  The
lstat()-based code has handling for that.

This also reverts 4fc6b6ee on master only.  That was done because
lstat() didn't previously work for symlinks (junction points), but now
it does.

Tested-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com
(cherry picked from commit 5fc88c5d53e43fa7dcea93499d230a0bf70f4f77)

Author: Thomas Munro <tmunro@postgresql.org>
Author: Alexandra Wang <alexandra.wang.oss@gmail.com>

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ca9921936ea352df30db2955fb2662695d8377be
Author: Thomas Munro <tmunro@postgresql.org>

Modified Files
--------------
src/backend/commands/tablespace.c    |  7 +------
src/backend/replication/basebackup.c | 12 +-----------
src/backend/storage/file/fd.c        |  5 -----
src/backend/utils/adt/misc.c         |  7 -------
src/bin/pg_checksums/pg_checksums.c  |  4 ----
src/bin/pg_rewind/file_ops.c         |  4 ----
src/common/file_utils.c              | 23 -----------------------
src/include/port.h                   |  1 -
src/include/port/win32_port.h        |  1 -
src/port/dirmod.c                    | 16 ----------------
10 files changed, 2 insertions(+), 78 deletions(-)