ppd-cache.c: Fix generating cupsUrfSupported (fixes #952)

[zdohnal]

The PPD keyword got the option values concatenated together without commas, which broke filter processing in libcupsfilters

Sometimes more commas better commas :)

[zdohnal]

I would say we could drop it once libcupsfilters gets a new release, which includes the fix which this keyword was introduced for in the first place.