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

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The abstract loses its odd capitalization
("Display and process credentials."). --help now goes through the pager,
like other commands with --no-pager.

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

@@ -28,6 +28,7 @@ INTROSPECTABLE=(
     systemd-cgls
     systemd-cgtop
     systemd-clonesetup
+    systemd-creds
     systemd-delta
     systemd-detect-virt
     systemd-dissect