report: 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.

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

@@ -62,6 +62,7 @@ INTROSPECTABLE=(
     systemd-pcrextend
     systemd-pty-forward
     systemd-random-seed
+    systemd-report
     systemd-socket-activate
     systemd-socket-proxyd
     systemd-stdio-bridge