Unicode validation for Posix

[Vino]

Hi All,

     As per the documentation from std.process it states that an 
exception is thrown if the variable contains invalid UTF-16 
characters and it can also be validated using "validate" function 
from std.utf, so the question is do we have a similar one for 
Posix as this seem to be applicable only to Windows.

```
Exception if the environment variable does not exist, or 
std.utf.UTFException if the variable contains invalid UTF-16 
characters (Windows only).
```

From,
Vino