Opening an optical drive device node without O_NONBLOCK autocloses the
tray, we run vol_id on every media change by kernel emitted "change"
events, which can make it hard to change the media when the tray closes
immediatey again.:) We check for cdrom_id to indicate an existing track,
if no media is found, we will not open the device with vol_id.
Thanks to Christian Krause and DavidZ for debugging and testing.
None of these rules is supposed to be changed by users, so move
them out of /etc. Custom rules, and automatically generated rules
stay in /etc. All rules are still processed in lexical order,
regardless which directory they live in.
