When we run tailscled under systemd, restarting the unit kills all child
processes, including "tailscale update". And during update, the package
manager will restart the tailscaled unit. Specifically on Debian-based
distros, interrupting `apt-get install` can get the system into a wedged
state which requires the user to manually run `dpkg --configure` to
recover.
To avoid all this, use `systemd-run` where available to run the
`tailscale update` process. This launches it in a separate temporary
unit and doesn't kill it when parent unit is restarted.
Also, detect when `apt-get install` complains about aborted update and
try to restore the system by running `dpkg --configure tailscale`. This
could help if the system unexpectedly shuts down during our auto-update.
Fixes https://github.com/tailscale/corp/issues/15771
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
955e2fcbfb