udevadm verify: introduce --root option

When udevadm verify is invoked without positional arguments and loads
all rules files from the system like the udev daemon does, this option
can be used to operate on files underneath the specified root path.

shell-completion/bash/udevadm

@@ -64,7 +64,7 @@ _udevadm() {
         [MONITOR_ARG]='-s --subsystem-match -t --tag-match'
         [TEST]='-a --action -N --resolve-names'
         [TEST_BUILTIN]='-a --action'
-        [VERIFY]='-N --resolve-names'
+        [VERIFY]='-N --resolve-names --root'
         [WAIT]='-t --timeout --initialized=no --removed --settle'
         [LOCK]='-t --timeout -d --device -b --backing -p --print'
     )
@@ -254,6 +254,10 @@ _udevadm() {
                     -N|--resolve-names)
                         comps='early never'
                         ;;
+                    --root)
+                        comps=$(compgen -A directory -- "$cur" )
+                        compopt -o dirnames
+                        ;;
                     *)
                         comps=''
                         ;;