Mark std.file.write and append as trusted

On Posix systems, they are safe because `writeImpl` is safe.
On Windows systems, they include unsafe functions `CreateFileW`, `CloseHandle`, `SetFilePointer` (only `append`) and `WriteFile` but we can verify they can be trusted.

I also change expressions like `WriteFile(...) == 1` to `WriteFile(...) != 0` because the [document of `WriteFile`](http://msdn.microsoft.com/en-us/library/windows/desktop/aa365747(v=vs.85).aspx) says it returns nonzero, not one.
I think comparing to one is already safe but it is clearer to be safe to follow the document.