pgsql: Provide lstat() for Windows.

Provide lstat() for Windows.

Junction points will be reported with S_ISLNK(x.st_mode), simulating
POSIX lstat(). stat() will follow pseudo-symlinks, like in POSIX (but
only one level before giving up, unlike in POSIX).

This completes a TODO left by commit bed90759fcb.

Tested-by: Andrew Dunstan <andrew(at)dunslane(dot)net> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com
(cherry picked from commit c5cb8f3b770c043509b61528664bcd805e1777e6)

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

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b73c1496dc7aba7748eb3d55b29c4eb0dc788296
Author: Thomas Munro <tmunro(at)postgresql(dot)org>

Modified Files
--------------
src/include/port/win32_port.h | 18 ++++++-
src/port/win32stat.c | 110 ++++++++++++++++++++++++++++++++++++++++--
2 files changed, 123 insertions(+), 5 deletions(-)