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(at)dunslane(dot)net>
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com
(cherry picked from commit 5fc88c5d53e43fa7dcea93499d230a0bf70f4f77)

Author: Thomas Munro <tmunro(at)postgresql(dot)org>
Author: Alexandra Wang <alexandra(dot)wang(dot)oss(at)gmail(dot)com>

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ca9921936ea352df30db2955fb2662695d8377be
Author: Thomas Munro <tmunro(at)postgresql(dot)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(-)