Add support for multiple system profiles

‘nixos-rebuild’ now accepts an argument ‘--profile-name’ (or ‘-p’),
denoting the name of a system profile to use.  The default is
‘system’, which maps to /nix/var/nix/profiles/system.  Any other value
maps to /nix/var/nix/profiles/system-profiles/<name>.  The GRUB menu
generator makes all system profiles available as submenus.  For
instance, doing

  $ nixos-rebuild boot -p test

will cause a menu named ‘NixOS - Profile 'test'’ to be added to the
GRUB boot menu, leaving the default system profile unaffected.

This is only supported for GRUB 2.