Fix podman leaking conmon processes

When running in the background without a full-blown init system,
`podman system service` will leak `conmon` processes for every
gitlab-runner job that executes via the docker socket API.  These
`conmon` processes almost immediately becomes zombies, and are never
cleaned up.  Eventually the zombies will consume all available PIDs.

Many attempts to fix this in various ways have all failed.  In all cases
the GitLab Runner process will start behaving strangely (or fail
completely) after an amount of time dependent on its usage executing
jobs.

Fix this by entirely reimplementing *pipglr* to utilize systemd and a
pair of lingering user-slices.  One for podman, another for the gitlab
runner.  Include a systemd timer service to affect runner cleanup,
periodically. Also update documentation and examples accordingly.

Signed-off-by: Chris Evich <chris_gitlab@icuc.me>

podman.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Podman API Service
+Requires=podman.socket
+After=podman.socket
+Documentation=man:podman-system-service(1)
+StartLimitIntervalSec=0
+
+[Service]
+Delegate=true
+Type=exec
+KillMode=process
+Environment=LOGGING="--log-level=warn"
+ExecStart=/usr/bin/podman $LOGGING system service
+StandardOutput=journal+console
+StandardError=inherit
+
+[Install]
+WantedBy=default.target