Closes#234802
The problem here is that with e.g.
{
uptime.prefix = "Up";
banner.command = "hostname | figlet -f slant";
}
`banner` still appears before `uptime` in the final motd text because
Nix sorts attribute names alphabetically internally.
To work around this without breaking compatibility or losing the
property to override individual sections in other modules - e.g.
{
banner.color = mkForce "blue";
}
I decided to introduce an option `priority` here, similar to the
priority field for `nginx`[1] and with the same semantics (i.e. higher
value means lower priority).
Internally a bunch of env vars are generated, i.e. `env0` to `envN` for
`N` sections with each of them containing a declaration for the TOML,
i.e. `env0` contains `{ uptime.prefix = "Up"; }` and `env1` contains
`{ banner.command = "hostname | figlet -f slant"; }` if `uptime.priority`
is set to a value below 1000.
In this order, the declarations are concatenated together by `jq(1)`
which doesn't sort keys alphabetically which results in a JSON
representation with `uptime` before `banner`. This is finally piped to
`json2toml` which converts this into TOML for rust-motd.
[1] https://nixos.org/manual/nixos/unstable/options#opt-services.nginx.virtualHosts._name_.locations._name_.priority
Makes it easier to configure `rust-motd`. Currently, it takes care of
the following things:
* Creating a timer to regularly refresh the `motd`-text and a hardened
service (which is still root to get access to e.g. fs-mounts, but
read-only because of hardening flags).
* Disabling `PrintLastLog` in `sshd.conf` if the last-login feature of
`rust-motd` is supposed to be used.
* Ensure that the banner is actually shown when connecting via `ssh(1)`
to a remote server with this being enabled.
