cdrom.eclass: Rename CDROM_NAME_SET array to CDROM_NAMES

cdrom.eclass: Rename CDROM_NAME_SET array to CDROM_NAMES

vapier seemed confused about what he wanted this variable to do as can
be seen in bug #139196. The eclass used it for the names of each disc,
regardless of the set, while ebuilds used it for the name of each
single-disc set. This was not helped by the fact that the set feature
has been totally undocumented. The former behaviour makes more sense
so let's rename the array to something less confusing.

This will not break ebuilds already using CDROM_NAME_SET. As they all
use just a single disc, they currently do not display the names given
in this variable anyway.