* Do not create stuff in localstatedir when doing `make install'
(since we may not have write access). In general, installation of
constant code/data should be separate from the initialisation of
mutable state.
default -> default-94-link
default-82-link -> /nix/store/cc4480...
default-83-link -> /nix/store/caeec8...
...
default-94-link -> /nix/store/2896ca...
experimental -> experimental-2-link
experimental-1-link -> /nix/store/cc4480...
experimental-2-link -> /nix/store/a3148f...
* `--profile' / `-p' -> `--switch-profile' / `-S'
* `--link' / `-l' -> `--profile' / `-p'
* The default profile is stored in $prefix/var/nix/profiles.
$prefix/var/nix/links is gone. Profiles can be stored anywhere.
* The current profile is now referenced from ~/.nix-profile, not
~/.nix-userenv.
* The roots to the garbage collector now have extension `.gcroot', not
`.id'.
