dumpkeys: option to list unassigned scancodes

To enable some keys on multimedia keyboards it is required to follow instructions on https://help.ubuntu.com/community/MultimediaKeys and search output of dumpkeys to find unassigned scancodes. Unfortunately, the list is huge and it's not completely evident what to look for.

It would be cool if dumpkeys could supply info on unassigned keys in convenient form.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: kbd 1.15-1ubuntu5
ProcVersionSignature: Ubuntu 2.6.38-11.48-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
CheckboxSubmission: b346f016d264f3ef62407d5e29c198a2
CheckboxSystem: daed2f3d6643b4a84b4520a2427f8c2b
Date: Thu Sep 1 13:08:48 2011
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: kbd
UpgradeStatus: Upgraded to natty on 2011-04-02 (151 days ago)