utmpdup: Ensure flushing when using follow flag

utmpdup: Ensure flushing when using follow flag

The following usages of utmpdump result in no output being flushed to the specified file because the default output buffering is fully buffered:

$ utmpdump --follow --output myOutputFile /var/log/utmp
$ utmpdump --follow /var/log/utmp > myOutputFile

This change configures line buffering for these scenarios so that output will be flushed after each log event.

Signed-off-by: Andrew Shapiro <anshapiro@gmail.com>