1.0.42.43: FD-STREAMS no longer hook into SERVE-EVENT by default

1.0.42.43: FD-STREAMS no longer hook into SERVE-EVENT by default

 * SOCKET-MAKE-STREAM, and MAKE-FD-STREAM have new keyword
   argument :SERVE-EVENTS which requests that blocking IO on the
   stream should dispatch to SERVE-EVENT. For SOCKET-MAKE-STREAM the
   default is T, for MAKE-FD-STREAM the default it NIL.

 * Don't call SYSREAD-MAY-BLOCK-P at all unless we need to to handle
   events or check for timeout.

 * Make WAIT-UNTIL-FD-USABLE use UNIX-SIMPLE-POLL instead of going
   into SUB-SERVE-EVENT when appropriate:

   ** Explicit requests to not serve events.

   ** Timeout 0.

   ** No other handlers and no periodic polling function.

 * When FD-STREAM-SERVE-EVENTS is false but write returns EWOULDBLOCK,
   don't queue output but wait till poll(2) says we can go.

 * UNIX-SIMPLE-POLL uses poll() only on platforms where a build-time
   test shows it to exist and work as expected. Elsewhere it is built
   on top of good 'ol select().