journalctl: add --list-invocations command and -I/--invocation options

The --list-invocations command is similar to --list-boots, but shows
invocation IDs of specified unit. This should be useful when showing
a specific invocation of a unit.

The --invocation option is similar to --boot, but takes a invocation ID
or an offset. The -I option is equivalent to --invocation=0.

shell-completion/bash/journalctl

@@ -47,11 +47,11 @@ _journalctl() {
                       --show-cursor --dmesg -k --pager-end -e -r --reverse
                       --utc -x --catalog --no-full --force --dump-catalog
                       --flush --rotate --sync --no-hostname -N --fields
-                      --list-namespaces'
+                      --list-namespaces --list-invocations -I'
         [ARG]='-b --boot -D --directory --file -F --field -t --identifier
                       -T --exclude-identifier --facility -M --machine -o --output
                       -u --unit --user-unit -p --priority --root --case-sensitive
-                      --namespace'
+                      --namespace --invocation'
         [ARGUNKNOWN]='-c --cursor --interval -n --lines -S --since -U --until
                       --after-cursor --cursor-file --verify-key -g --grep
                       --vacuum-size --vacuum-time --vacuum-files --output-fields'