systemctl: add new option to mount image inside a running service namespace

Use the new DBUS method and follow the same pattern as the
systemctl bind command.

shell-completion/bash/systemctl.in

@@ -214,7 +214,7 @@ _systemctl () {
                              list-timers list-units list-unit-files poweroff
                              reboot rescue show-environment suspend get-default
                              is-system-running preset-all'
-        [FILE]='link switch-root bind'
+        [FILE]='link switch-root bind mount-image'
         [TARGETS]='set-default'
         [MACHINES]='list-machines'
         [LOG_LEVEL]='log-level'

shell-completion/zsh/_systemctl.in

@@ -32,6 +32,7 @@
         "list-dependencies:Show unit dependency tree"
         "clean:Remove configuration, state, cache, logs or runtime data of units"
         "bind:Bind mount a path from the host into a unit's namespace"
+        "mount-image:Mount an image from the host into a unit's namespace"
     )
 
     local -a machine_commands=(
@@ -383,6 +384,10 @@ done
         _files
     }
 
+(( $+functions[_systemctl_mount-image] )) || _systemctl_mount-image() {
+        _files
+    }
+
 # no systemctl completion for:
 #    [STANDALONE]='daemon-reexec daemon-reload default
 #                  emergency exit halt kexec list-jobs list-units