1.0.48.7: add FD-STREAM-FD-TYPE, use it to decide when to poll the fd

1.0.48.7: add FD-STREAM-FD-TYPE, use it to decide when to poll the fd

  1.0.42.43 introduced a regression that caused reads from non-regular files
  to potentially block in an uninterruptible state, as WAIT-UNTIL-FD-USABLE
  started being called only when there was a timeout or events to serve.

  This was a win for regular files, because filling the input buffer then
  required one syscall less.

  ...but since uninterruptible blocking reads aren't fun for anyone, let's
  make things a bit smarter:

  * When creating an FD-STREAM, use fstat() to get the type of the fd, and
    store a keyword describing it in the FD-STREAM.

  * REFILL-INPUT-BUFFER now calls SYSREAD-MAY-BLOCK-P always if the file
    anything but :REGULAR -- so pipes and sockets and whatnot get their
    select() calls, and a read from a stream to one can no longer cause
    SBCL to hang.