execute: Add new PassEnvironment= directive This directive allows passing environment variables from the system manager to spawned services. Variables in the system manager can be set inside a container by passing `--set-env=...` options to systemd-spawn. Tested with an on-disk test.service unit. Tested using multiple variable names on a single line, with an empty setting to clear the current list of variables, with non-existing variables. Tested using `systemd-run -p PassEnvironment=VARNAME` to confirm it works with transient units. Confirmed that `systemctl show` will display the PassEnvironment settings. Checked that man pages are generated correctly. No regressions in `make check`.

shell-completion: systemd-run: add the property RuntimeDirectory

shell-completion: systemd-run: add the property ProtectHome

shell-completion: systemd-run: add the property ProtectSystem

shell-completion: systemd-run: add new property EnvironmentFile

Add zsh completion on systemd-run's properties

zsh-completion: add missing completions for systemd-run

zsh-completion: Move machine listing to autoload Instead of having two different listings of machines, use an autoloaded function that can be used by other shell completions in the future. It will also allow editing a single file to change the way machinectl and systemd-run completion for machines.

zsh-completion: add missing options for systemd-run

zsh-completion: add systemd-run