Bump the NixOS version number to 0.2

A new init system justifies a new version number!
Remove obsolete environment variables
Revert "Remove obsolete environment variables"

This reverts commit ac8080b83c8bcaaadd35f7430ccc0add0eade15a.
Remove bogus comment
Rename ‘boot.systemd’ to ‘systemd’

Suggested by Mathijs Kwik.  ‘boot.systemd’ is a misnomer because
systemd affects more than just booting.  And it saves some typing.
Rename ‘system.build.systemd’ to ‘systemd.package’

This makes it cheaper to test a new systemd and is more consistent
with other modules.
nixos-rebuild: Use ‘[ ... ]’ instead of ‘test’
Set the NixOS version to something useful when building from Git
Set some missing types
Add missing file
Fix ‘nixos-rebuikd dry-run’
Ignore .version-suffix file created by nixos-rebuild
Rewrite NetworkManager job to systemd service
Rename NetworkManager-init service to networkmanager-init