Remove use of nixops-specific 'keys' group and key services.
Instead:
- Add nix-bitcoin-secrets.target, which should be required by all
units that depend on secrets. (To keep it simple, it's okay to meet
the secrets dependency indirectly by e.g. depending on bitcoind.)
Various secret deployment methods can use this target by
setting up the secrets before activating the target.
In case of nixops we just specify that nixops' keys.target comes
before nix-bitcoin-secrets.target.
If the target is left undefined in the case of manual secrets
deployment, systemd will simply ignore unit dependencies on
the target.
- Allow all users to access the secrets dir.
The access protection for the individual secret files is unchanged.
This allows us to drop the unit dependency on the nixops 'keys' group.
