NixOS option `security.hideProcessInformation` for globally restricting
access to /proc has been removed.
Use per-service restrictions via 'ProtectProc' instead.
Rename
`nix-bitcoin.security.hideProcessInformation` to
`nix-bitcoin.security.dbusHideProcessInformation`
because this option now only implements the dbus restriction.
Previously, systemd-status was broken for all users except root.
Use a 'default' deny policy, which is overridden for group 'proc'.
Add operator to group 'proc'.
Also, remove redundant XML boilerplate.
Previously, due to the dependency on a helper service, this dbus config
was initially inactive after system boot, allowing for unrestricted use
of the problematic dbus call.
This also broke the accompanying VM test on faster systems.
Remove 'allow' policy for root because it's a no-op:
1. It's overridden by the 'mandatory' deny policy.
2. Root can use all dbus calls anyways, regardless of policy settings.
Also, add some comments.
