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

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The abstract is changed to use the imperative
form ("Configure …" instead of "Configures …"), consistent with the
other commands.

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

@@ -16,6 +16,7 @@ INTROSPECTABLE=(
     systemd-ask-password
     systemd-backlight
     systemd-dissect
+    systemd-firstboot
     systemd-id128
     systemd-notify
     systemd-veritysetup