Fix oc stream seek

I was debugging a strange bug for fun in my vacation: https://github.com/owncloud/music/issues/212

In short fseek end ftell are broken when using the OC stream wrapper. The stream_seek implementation needs to return true or false, depending on the result of fseek, see http://de1.php.net/manual/de/streamwrapper.stream-seek.php#115111

test in ef6db5e
fix in 01e61bf

cc @icewind1991 @MorrisJobke 

@karlitschek I recommend backporting unless we want to hunt obscure problems in the enterprise edition