Make std.file.mkdir 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.CreateDirectoryW` and `std.internal.cstring.tempCStringW`

On Posix systems:
- use of unsafe functions `core.sys.posix.sys.stat.mkdir` and `std.internal.cstring.tempCString`

Additionally, I replaced `enforce` with `cenforce` to obtain error information on Windows systems.