Added the `std.file.getAvailableDiskSpace` functionality.

$(REF getAvailableDiskSpace, std,file) receives as a parameter the path of a file or
directory in the file system, and returns the available disk space on the mounted filesystem.
If the given path is nonexistent, an exception is thrown.

---
import std.file;
ulong size = getAvailableDiskSpace(".");
assert(size > 0);
---

---
import std.file;
assertThrown(getAvailableDiskSpace("NonExistentFile"));
---

