DSKDEAD: An optional S_SKIN Definition for a Sound to Play when a Racer Dies

ringracers_user-defined-death-sound_GlNCVKGNbv

I don't think there's much more to be said of this MR than the title at least until I get some feedback of course. Whilst by-and-large this is intended as a by-the-books implementation of an extra sound lump for an S_SKIN definition to require, it differs from existing sound lumps for S_SKIN in that this lump is handled as optional rather than left to fall back upon the DSTHOK sound lump as one can prove for one's self by setting DSKDEAD to DSTHOK and observing that only when intentionally set as such will DSTHOK be permissable for use as the defined sound lump. The major benefit of this is that all existing racer WADs of course will have to do nothing to accomodate this MR, it is simply an option open for future racer WADs to add some more character to the death sequence beyond what my other MRs already aim to provide.

Hope you guys (Kart Krew & associated repo managers) like it!