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.

man/systemd-cgls.xml

@@ -31,7 +31,7 @@
       <command>systemd-cgls</command>
       <arg choice="opt" rep="repeat">OPTIONS</arg>
       <arg choice="plain"><option>--unit</option>|<option>--user-unit</option></arg>
-      <arg choice="opt" rep="repeat">UNIT</arg>
+      <arg choice="plain" rep="repeat">UNIT</arg>
     </cmdsynopsis>
   </refsynopsisdiv>