pgsql: Make unlink() work for junction points on Windows.

Make unlink() work for junction points on Windows.

To support harmonization of Windows and Unix code, teach our unlink()
wrapper that junction points need to be unlinked with rmdir() on
Windows.

Tested-by: Andrew Dunstan <andrew(at)dunslane(dot)net>
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com
(cherry picked from commit f357233c9db8be2a015163da8e1ab0630f444340)

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
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https://git.postgresql.org/pg/commitdiff/b9d4a927d627bf5802e7e8e797ffc60d21bbac8b
Author: Thomas Munro <tmunro(at)postgresql(dot)org>

Modified Files
--------------
src/port/dirmod.c | 28 +++++++++++++++++++++++++++-
1 file changed, 27 insertions(+), 1 deletion(-)