Use explicit fflush() instead of setvbuf()

Flushing after every line when printing to console is desirable when
running with systemd but setvbuf() has slightly different semantics
on Windows that causes warnings.  Just do an explicit fflush() after
each line print to console instead.
Merge pull request #4892

e982b57 Use explicit fflush() instead of setvbuf() (Adam Weiss)