Filesystem API: Prevent an endless self-calling loop in wp_tempnam().
Under certain conditions upgrades on Windows may fail because wp_tempnam() gets called in a loop.
This can happen when wp_tempnam() is called with \.maintenance for the $filename parameter. The function strips the extension, in this case .maintenance, which results in an empty filename. Because it's empty, wp_tempnam() calls itself with dirname( '\.maintenance' ). On *nix systems this would be "/" which allows wp_tempnam() to fall back on time(). But on Windows it's "\".
This change adds the backslash to the list of characters which allow wp_tempnam() to fall back on time().