Make stderr fully buffered on Windows to improve named performance when
it is logging to stderr, which happens e.g. in system tests. Note that:
- line buffering (_IOLBF) is unavailable on Windows,
- fflush() is called anyway after each log message gets written to the
default stderr logging channels created by libisc.
(cherry picked from commit c72da3497d)
f1b89d0d9f