Double file cache fix. Fixes ticket #15500

Double caching for e.g. dvdplayer has been broken for a long time. While debugging this problem (which took quite some time) I also noticed that CurlFile didn't return the proper (read: most optimal) chunksize, furthermore FileCache never reported cache full. The actual problem for double cache was that when the cache was full, we still kept reading from the source which get disposed due to seek-requests which in turn also caused seek backs in the stream (which are really slow on e.g. http). This revealed itself while playing mp4 files with high a/v offset but generally this should improve performance (especially on slow systems) for all cached files.

Would be great if this could be included for Helix.
Agreed. Renamed it to CanCacheWrite(int). You ok with this?