Replaced snprintf and vnsprintf calls by dpsnprintf and dpvsnprintf calls, to ensure...

Replaced snprintf and vnsprintf calls by dpsnprintf and dpvsnprintf calls, to ensure coherence among the various platforms (the Win32 versions of those functions aren't C99 compatible). dp(v)snprintf functions return -1 when the buffer is too small to contain all the data, and insure null termination.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4991 d7cf8633-e32d-0410-b094-e92efae38249