containers: systemd exits with non-zero code When a systemd service running in a container exits with a non-zero code, it can be useful to terminate the container immediately and get the exit code back to the host, when systemd-nspawn returns. This was not possible to do. This patch adds the following to make it possible: - Add a read-only "ExitCode" property on PID 1's "Manager" bus object. By default, it is 0 so the behaviour stays the same as previously. - Add a method "SetExitCode" on the same object. The method fails when called on baremetal: it is only allowed in containers or in user session. - Add support in systemctl to call "systemctl exit 42". It reuses the existing code for user session. - Add exit.target and systemd-exit.service to the system instance. - Change main() to actually call systemd-shutdown to exit() with the correct value. - Add verb 'exit' in systemd-shutdown with parameter --exit-code - Update systemctl manpage. I used the following to test it: | $ sudo rkt --debug --insecure-skip-verify run \ | --mds-register=false --local docker://busybox \ | --exec=/bin/chroot -- /proc/1/root \ | systemctl --force exit 42 | ... | Container rkt-895a0cba-5c66-4fa5-831c-e3f8ddc5810d failed with error code 42. | $ echo $? | 42 Fixes https://github.com/systemd/systemd/issues/1290