udev/iocost: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added. The tool has no man page of its own, so
iocost.conf(5), which describes it, is referenced. The test now also looks
for commands in /usr/lib/udev, where the udev helpers are installed.

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

@@ -3,9 +3,9 @@
 set -eux
 set -o pipefail
 
-# Some commands are in $PATH, others are in our private directory.
-# Check the other location too.
-PATH=$PATH:/usr/lib/systemd
+# Some commands are in $PATH, others are in our private directories.
+# Check the other locations too.
+PATH=$PATH:/usr/lib/systemd:/usr/lib/udev
 
 export SYSTEMD_PAGER=cat
 
@@ -17,6 +17,7 @@ INTROSPECTABLE=(
     homectl
     hostnamectl
     importctl
+    iocost
     journalctl
     localectl
     loginctl