shell-completion/zsh: silence error when machinectl is not installed

shell-completion/zsh: silence error when machinectl is not installed

This fixes a few unrelated issues:
- when ENABLE_MACHINED is false, machinectl is not installed, but _sd_machines
  is still used in a few places that want to complete -M and such.
  Also, bash completion calls machinectl in various places.
  Make missing machinectl mean "no machines" in this case, so
  that no error is generated in the callers.
- machinectl list --full would print multiple lines of output per machine,
  breaking grep, issue introduced in e2268fa43742ece4a5cdc2e93f731b2bb2fcc883.
  Using --max-addresses=1 would fix the issue, but let's use
  --max-addresses=0 because we now can.
- the lists used in various places were slightly different for no good reason.
- don't use a subshell if not necessary.

The code for bash still uses the same combined list of images and running
machines for various commands. The zsh code uses images for start/clone, and
running machines for the rest. Maybe something to fix in the future.

Replaces #25048.