Fix tmpdir issues with commit e213de8e78

Fix tmpdir issues with commit e213de8e78

Commit e213de8e78 fixed a problem with path lengths to a tempdir on
Windows, but caused problems on at least some Unix systems where the
system tempdir is on a different file system. To work around this, only
used the system temdir for the destination of pg_replslot on Windows,
and otherwise restore the old behaviour.

Backpatch to relase 14 like the previous patch.

Problem exposed by a myriad of buildfarm animals.