client: Explicitly systemctl start rpm-ostreed if root, dump status

[cgwalters]

This is a hackier an alternative to #2932
that we can ship immediately because we won't block on SELinux policy.

For historical reasons, the daemon ends up doing a lot of
initialization before even claiming the DBus name. For example,
it calls ostree_sysroot_load(). We also end up scanning
the RPM database, and actually parse all the GPG keys
in /etc/pki/rpm-gpg etc.

This causes two related problems:

• By doing all this work before claiming the bus name, we
  race against the (pretty low) DBus service timeout of 25s.
• If something hard fails at initialization, the client can't
  easily see the error because it just appears in the systemd
  journal. The client will just see a service timeout.

By explicitly using systemctl start rpm-ostreed.service,
systemd does all of the error checking for us without involving
dbus-broker as a middleman.

Further, by using systemctl status rpm-ostreed.service on
failure, we reuse systemd's nice rendering of the status
of the unit instead of reinventing our own.

This PR effectively replicates openshift/machine-config-operator#2642
in our code.

[cgwalters]

Demo:

[root@cosa-devsh ~]# systemctl stop rpm-ostreed
[root@cosa-devsh ~]# umount /boot
[root@cosa-devsh ~]# rpm-ostree status
Job for rpm-ostreed.service failed because the control process exited with error code.
See "systemctl status rpm-ostreed.service" and "journalctl -xeu rpm-ostreed.service" for details.
× rpm-ostreed.service - rpm-ostree System Management Daemon
     Loaded: loaded (/usr/lib/systemd/system/rpm-ostreed.service; static)
     Active: failed (Result: exit-code) since Tue 2021-06-29 20:55:44 UTC; 11ms ago
       Docs: man:rpm-ostree(1)
    Process: 1342 ExecStart=/usr/bin/rpm-ostree start-daemon (code=exited, status=1/FAILURE)
   Main PID: 1342 (code=exited, status=1/FAILURE)
     Status: "error: Couldn't start daemon: Error setting up sysroot: loading sysroot: Unexpected state: /run/ostree-booted found, but no /boot/loader directory"
        CPU: 32ms

Jun 29 20:55:44 cosa-devsh systemd[1]: Starting rpm-ostree System Management Daemon...
Jun 29 20:55:44 cosa-devsh rpm-ostree[1342]: Reading config file '/etc/rpm-ostreed.conf'
Jun 29 20:55:44 cosa-devsh rpm-ostree[1342]: error: Couldn't start daemon: Error setting up sysroot: loading sysroot: Unexpected state: /run/ostree-booted found, but no /boot/loader directory
Jun 29 20:55:44 cosa-devsh systemd[1]: rpm-ostreed.service: Main process exited, code=exited, status=1/FAILURE
Jun 29 20:55:44 cosa-devsh systemd[1]: rpm-ostreed.service: Failed with result 'exit-code'.
Jun 29 20:55:44 cosa-devsh systemd[1]: Failed to start rpm-ostree System Management Daemon.
error: exit code: 1
[root@cosa-devsh ~]# 

[cgwalters]

(Added a test)

[jlebon]

Nice! This makes a lot of sense to me, and I think it's a good simple alternative to #2932.