1.0.8.16: refactored fd-stream buffering

1.0.8.16: refactored fd-stream buffering

 Thanks to David Smith and Andreas Bogkt who diagnosed the memory
 leaks this patch fixes.

 * Instead of having FD-STREAM objects directly hold onto input and
   output buffer SAPs and head/tail indexes, use BUFFER objects which
   contain the SAP, size of the memory area, head/tail indexes, and
   are have finalizers to deallocate the system memory assosicated
   with the SAP. (This fixes system memory leaks when streams are not
   properly closed.)

 * Make CLOSE :ABORT release the output queue associated with the
   stream. (This was another memory leak in the old system: now
   the finalizers make not doing this safe, but it's still better
   to recycle the buffers.)

 * Slightly reduce lock contention by grabbing the *AVAILABLE-BUFFERS*
   lock only if there is something there right before the lock is
   taken, and by doing allocation outside the lock.

 * Rename and refactor FROB-OUTPUT and friends:

     BUFFER-OUTPUT is the main interface function, which always
       adds new output to the current buffer / output queue.

     WRITE-OR-BUFFER-OUTPUT tries to write immediately, falling
       back to buffering if writing is not possible.

     WRITE-OUTPUT-FROM-QUEUE is called by the SERVE-EVENT
       system to deal with output queue.

     FLUSH-OUTPUT-BUFFER writes the current buffer out if possible,
       queues it otherwise. Ensures that the output buffer of
       the stream is empty on return (and returns that buffer).

 * Deprecate SB-SYS:OUTPUT-RAW-BYTES. There doesn't seem to be any
   real reason to export this kind of stuff.

 * Increment the fasl version.