Always line-buffer bup restore stdout

Flush stdout after every line, even when stdio is not a tty, to
provide more regular progress information.  Useful for progress
monitoring by a parent process or when watching a logfile with "tail".

Signed-off-by: Simon Persson <simonpersson1@gmail.com>
[rlb@defaultvalue.org: adjust commit message]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>