The canUpdateStatus check was simplified in #2608, but it accidentally flipped
the condition. The correct check is as follows: If the output is a pipe then
restic probably runs in mintty/cygwin. In that case it's possible to
update the output status. In all other cases it isn't.
This commit inverts to condition again to offer the previous and correct
behavior.
On shutdown the backup commands waits for the terminal output goroutine
to stop. However while running in the background the goroutine ignored
the canceled context.
The previous implementation was repeating the implementation that is
found inside of io.WriteString. Simplify by making use of the stdlib's
implementation.
This now keeps the cursor at the first column of the first status line
so that messages printed to stdout or stderr by some other part of the
progarm will still be visible. The message will overwrite the status
lines, but those are easily reprinted on the next status update.
