repart: 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

@@ -73,6 +73,7 @@ INTROSPECTABLE=(
     systemd-pcrlock
     systemd-pty-forward
     systemd-random-seed
+    systemd-repart
     systemd-report
     systemd-sbsign
     systemd-sleep