Rewrite chunk queueing.

Rewrite chunk queueing.

Since paraslash-0.2.16 the audio file is mmapped, so it is no longer neccessary
to make a copy of the chunk data in case a chunk can not be sent immediately
because the write would block. Just remember the chunk number and the number of
bytes of that chunk which were sent (non-zero in case of short writes).

The chunk data can then be retrieved by the new vss_get_chunk() function.