Fix support for multi-disc sets in cue files

Allows both DISC and DISCNUMBER to be used for disc number.

Allows DISCC, DISCTOTAL, or TOTALDISCS to be used for total
number of discs in the set.

Permits quotes to be optional as disc numbers might not have them.

Reference:

http://bugs.slimdevices.com/show_bug.cgi?id=17770