Fixed dirpath generation on OSX with Filesystem driver

Since 43729fb, Filesystem driver checks path length for Windows
(limited to 260 chars by Microsoft). However, Windows check is made
with `stristr(PHP_OS,'WIN')`, which makes it search for `WIN` string
in all `PHP_OS`. This makes it fail on OSX, where `PHP_OS` == `Darwin`.

This patch checks if `WIN` is at the beginning of `PHP_OS` string.
According to [PHP doc](http://fr2.php.net/php_uname),
value on Windows can be `WIN32`, `WINNT` or `Windows`.
Merge pull request #134 from lolautruche/fixFilesystemOSXRegression

Fixed dirpath generation on OSX with Filesystem driver