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

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. --help now goes through the pager, like other
commands with --no-pager.

Synopses are adjusted. --user-unit/--unit without any unit names is
technically allowed, but not very meaningful, since it's the same as
the default mode. So [] is dropped. And the prog help string is then
adjusted to match the man page.

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

@@ -19,6 +19,7 @@ INTROSPECTABLE=(
     systemd-binfmt
     systemd-bsod
     systemd-cat
+    systemd-cgls
     systemd-dissect
     systemd-firstboot
     systemd-hwdb