if the ringbuf is shared among multiple threads (e.g. the sbbs->outbuf is
shared between output_thread() and passthru_thread()) - it was possible
for a race condition to occur between the caller would call RingBufFree
to determine the available space in the ringbuf and the call to RingBufWrite
which would happily overflow the allocated buffer if more data was
written to the ringbuf (by another thread) in the unprotected time between the
RingBufFree and RingBufWrite calls.

Now, RingBufWrite() can perform short-writes and will return a length less
than what was requested to write when there is not enough available space
to write the requested length.

Hopefully this resolves the corruption/crash issue Deuce is seeing in
sbbs's passthru_thread().