mingw: support Windows Server 2016 again

mingw: support Windows Server 2016 again

It was reported to the Git for Windows project that a simple `git init`
fails on Windows Server 2016:

  D:\Dev\test> git init
  error: could not write config file D:/Dev/test/.git/config: Function not implemented
  fatal: could not set 'core.repositoryformatversion' to '0'

According to https://endoflife.date/windows-server, Windows Server 2016
is officially supported for another one-and-a-half years as of time of
writing, so this is not good.

The culprit is the `mingw_rename()` changes that try to use POSIX
semantics when available, but fail to fall back properly on Windows
Server 2016.

This fixes https://github.com/git-for-windows/git/issues/5695.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>