FileStream improvement in Win32

[TsudaKageyu]

Made some improvements on FileStream in Win32 for reducing the resource consumption.

[lalinsky]

I'm not very familiar with the native Windows file API. What are the advantages?

[TsudaKageyu]

In Win32, fopen() creates a file descriptor in addition to a native file handle. It significantly increases resource consumption.
CreateFile can open virtually unlimited files concurrently, but fopen() can open up to a couple of thousands files.

[lalinsky]

Ah, thanks.