id128: add 'var-partition-uuid' verb

As per DPS the UUID for /var/ should be keyed by the local machine-id,
which is non-trivial to do in a script. Enhance 'systemd-id128' to
take 'var-partition-uuid' as a verb, and if so perform the
calculation.

shell-completion/bash/systemd-id128

@@ -34,7 +34,7 @@ _systemd_id128() {
     )
 
     local -A VERBS=(
-        [STANDALONE]='new machine-id boot-id invocation-id help'
+        [STANDALONE]='new machine-id boot-id invocation-id var-partition-uuid help'
         [ARG]='show'
     )