Add 'nix upgrade-nix' command

This command upgrades Nix to the latest stable version by installing a
store path obtained from

  https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix

which is the same store path that the installer at
https://nixos.org/nix/install.sh uses.

The upgrade fails if Nix is not installed in a profile (e.g. on NixOS,
or when installed outside of the Nix store).

src/nix/command.hh

@@ -5,6 +5,8 @@
 
 namespace nix {
 
+extern std::string programPath;
+
 struct Value;
 class Bindings;
 class EvalState;