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

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. An explicit "Commands" option group is added
so that the section heading stays the same. The abstract is changed to the
imperative. --help now goes through the pager, like other commands with
--no-pager.

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

@@ -90,6 +90,7 @@ INTROSPECTABLE=(
     systemd-sysctl
     systemd-sysinstall
     systemd-sysupdate
+    systemd-sysusers
     systemd-tpm2-clear
     systemd-tpm2-setup
     systemd-tty-ask-password-agent