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 e2268fa437.
  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.

shell-completion/bash/systemd-nspawn

@@ -35,7 +35,9 @@ __get_slices() {
 
 __get_machines() {
     local a b
-    machinectl list --full --no-legend --no-pager | { while read a b; do echo " $a"; done; };
+    { machinectl list --full --max-addresses=0 --no-legend --no-pager 2>/dev/null; echo ".host"; } | \
+	{ while read a b; do echo " $a"; done; } | \
+        sort -u
 }
 
 __get_env() {