journalctl: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The description refers to the "journalctl"
option namespace; the varlink-mode namespace is not described.

test/units/TEST-74-AUX-UTILS.introspect-cli.sh

@@ -17,6 +17,7 @@ INTROSPECTABLE=(
     homectl
     hostnamectl
     importctl
+    journalctl
     localectl
     networkctl
     oomctl