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

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The "Options" group marker after --help/--version
is dropped, so that all options are listed in one section after the verbs.
The synopsis now uses the generic "[OPTION…] COMMAND …" form: a bare
VERSION is not accepted as the first argument, and the man page does not
list it either. --help now goes through the pager, like other commands
with --no-pager.

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

@@ -89,6 +89,7 @@ INTROSPECTABLE=(
     systemd-storagetm
     systemd-sysctl
     systemd-sysinstall
+    systemd-sysupdate
     systemd-tpm2-clear
     systemd-tpm2-setup
     systemd-tty-ask-password-agent