Make std.file.timeLastModified(in char[]) safe

On Windows systems, it contains the following unsafe operation:

- (Fix) use of void initializer for `SysTime`

On Posix systems, it contains the following unsafe operations but we can verify they can be trusted.

- use of `core.sys.posix.sys.stat.stat`
- use of `std.internal.cstring.tempCString` and `tempCString.Res.~this`
- taking an address of a local variable (`statbuf`)

Edit: I missed void initializer for `SysTime`.
I define `trustedVoidInit` to wrap it but I'm not sure it does not affect to the performance (void initializer is used for performance).