progress-bar: Add unbuffered variant

progress-bar: Add unbuffered variant

The progress_bar functions do their own buffering: they reconfigure
stderr, then print, then flush and disable buffering on their own. In
situations where multiple progress bars are being drawn at a time (for
example, in updatectl), it's even more efficient to hoist the buffering
and flushing to the call site, and avoid drawing each progress bar
individually.

To that end, new _unbuffered variants of the progress_bar functions. And
we use them in updatectl.