bcachefs: thread_with_stdio: convert to darray

- eliminate the dependency on printbufs, so that we can lift
thread_with_file for use in xfs
- add a nonblocking parameter to stdio_redirect_printf(), and either
block if the buffer is full or drop it on the floor - don't buffer
infinitely

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: factor out thread_with_file, thread_with_stdio

thread_with_stdio now knows how to handle input - fsck can now prompt to
fix errors.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>