Fixed dirpath generation on OSX with Filesystem driver
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`.

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