Make DRelease take an anonymous pointer

Make DRelease take an anonymous pointer

DRelease is used to free data blocks returned by the dir package. These
can be any of a number of different types, which we were previously
dealing with by casting everything to (struct buffer *) before calling
DRelease.

Instead, make DRelease take a (void *), so we can free any dir package
product without requiring casts.

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/159
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>