shell-completion: add systemd-cat --namespace=

Follow-up for 45bcab66a9

Addresses https://github.com/systemd/systemd/pull/31754#discussion_r1524715062

shell-completion/bash/systemd-cat

@@ -31,7 +31,7 @@ _systemd_cat() {
 
     local -A OPTS=(
         [STANDALONE]='-h --help --version'
-        [ARG]='-t --identifier -p --priority --stderr-priority --level-prefix'
+        [ARG]='-t --identifier -p --priority --stderr-priority --level-prefix --namespace'
     )
 
     _init_completion || return
@@ -47,6 +47,9 @@ _systemd_cat() {
             --level-prefix)
                 comps='yes no'
                 ;;
+            --namespace)
+                comps=$(journalctl --list-namespaces --output=cat 2>/dev/null)
+                ;;
         esac
         COMPREPLY=( $(compgen -W '$comps' -- "$cur") )
         return 0

shell-completion/zsh/_systemd

@@ -19,6 +19,7 @@ case "$service" in
             {-t+,--identifier=}'[Set syslog identifier.]:syslog identifier:' \
             {-p+,--priority=}'[Set priority value.]:value:({0..7})' \
             '--level-prefix=[Control whether level prefix shall be parsed.]:boolean:(1 0)' \
+            '--namespace=[Connect to specified journal namespace.]:journal namespace:' \
             ':Message'
         ;;
     systemd-cgls)