Fix DVD eject at shutdown (on some computers)

Two probably related 0.14 regressions:

• pressing the DVD reader eject button neither ejects the DVD, nor triggers the emergency shutdown procedure
• 0.14-rc1 - DVD won’t eject before wiping of RAM during shutdown process

It might be that the old inverted logic bug that forced us to pretend to lock the DVD eject button in order to effectively unlock it, was fixed. Who knows.

Indeed, linux 2.6.38-rc1 [[reworked disk event handling|http://lwn.net/Articles/423619/]]
and adds block.events_dfl_poll_msecs that can be used to set the
interval for polling block devices for events. This does not seem
to work properly though: settings it to 1000 makes it work in
VirtualBox with its virtual CD-ROM drive, but no value seems to work
on the one real machine I have available with a CD-ROM drive.

Related issues

• Related to #7194 (closed)

Original created by @tails on 5447 (Redmine)