socket-proxyd: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The synopsis no longer marks the target as
optional, since exactly one HOST:PORT or SOCKET argument is required.
The hint about systemd.time(7) is kept as the footer.

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

@@ -49,6 +49,7 @@ INTROSPECTABLE=(
     systemd-pcrextend
     systemd-pty-forward
     systemd-socket-activate
+    systemd-socket-proxyd
     systemd-tty-ask-password-agent
     systemd-veritysetup
     timedatectl