Now all services that access secrets only run after the secrets setup
has finished.
Previously, we assumed that the systemd `after` dependency is
transitive, i.e. that adding an `after = [ "bitcoind.service" ]`
to a service implicitly pulled in the `after` dependency to
`nix-bitcoin-secrets.target` (which is defined for `bitcoind`).
This is not the case. Services could start before secrets setup
had finished, leading to service failure.
This still hides the proc subdirectories for other processes.
Without this setting, fulcrum fails when the config value of
`fast-sync` is greater than 2^31 bytes.
