pretty-print: enable stdio buffering while outputting progress bar

pretty-print: enable stdio buffering while outputting progress bar

Writing the progress bar so far was irritatingly slow, which was caused
by the fact that the various things we output so far resulted in one
write() syscall each because STDERR is unbuffered by default.

Let's fix that, and temporarily turn on full buffering for stderr,
restoring the normal unbuffered output right after.

This makes progress bar print visibly more efficient (and flicker free
too, since terminals no longer will move the cursor around during
output).