[PATCH] Make refbufs use size_t for sizes instead of unsigned long

This may or may not solve any real issues on 64bit archs at the moment, but it surely is more correct and makes large parts of Icecast code more consistent, where adding casts would be necessary otherwise.

This is the first of a long series of patches that are supposed to enhance Icecast's maintainability, portability and correctness. They are the result of an ongoing (and by now week-long) audit that shows that there are subtle issues hidden in Icecast that only work by accident at this point -- fixing those will make it much easier on the long run to maintain Icecast's quality.