Make std.file.setAttributes safe

It contains the following unsafe operations but we can verify that they can be trusted.

On Windows systems:
- use of unsafe functions `core.sys.windows.windows.SetFileAttributesW` and `std.internal.cstring.tempCStringW`

On Posix systems:
- use of unsafe functions `core.sys.posix.sys.stat.chmod` and `std.internal.cstring.tempCString`