Update nix to 1.5.1
Refactorings to not repeat ourselves

ceh_path_prepend from now on defined in base.sh, together with CEH_NIX
and CEH_NIX_DOWNLOAD.  All interested scripts can go from there.
Check for /opt/ceh/home symlink in nix-init.sh
Update README.org
Warn the user of non-existing Nix in scripts/nix-profile.sh
Remove -x from scripts/nix-init.sh
Add scripts/nix-upgrade.sh