Coverity reported that the new `mingw_rename()` function may overrun the
`FileName` buffer if using a long path.
The reason is that I forgot to adjust it for long path support, and
while looking at this, I realized that I also had forgotten to adjust
`mingw_strbuf_realpath()` and `is_path_owned_by_current_sid()`, too.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>