sh: Some changes to stderr flushing:

sh: Some changes to stderr flushing:
* increase buffer size from 100 to 256 bytes
* remove implied flush from out2str(), in particular this avoids unnecessary
  flushing in the middle of a -x tracing line
* rename dprintf() to out2fmt_flush(), make it flush out2 and use this
  function in various places where flushing is desired after an error
  message