Pull up following revision(s) (requested by jmcneill in ticket #1656):
sbin/drvctl/drvctl.8: revision 1.11
sbin/drvctl/drvctl.8: revision 1.12
sbin/drvctl/drvctl.c: revision 1.11
sbin/drvctl/drvctl.c: revision 1.12
add an optional argument to the -p flag that lets you extract specific
property values from the command-line:
  $ drvctl -p wd0 disk-info/geometry/cylinders-per-unit
  620181
  $ drvctl -p wd0 device-driver device-unit
  wd
  0
  $ drvctl -p wd0 nonexistent || echo "not found"
  not found
add the -t option that modifies -l behaviour to recursively scan for
child devices and print them in tree format:
$ drvctl -lt usb0
uhub0
  uhub6
    uhidev0
      ukbd0
        wskbd1
    uhidev1
      ums0
        wsmouse1
      uhid0
    ubt0

(riz)