mount-util: fold what we need from get_mount_flags() bind_remount_one_with_mountinfo()

mount-util: fold what we need from get_mount_flags() bind_remount_one_with_mountinfo()

And get rid of get_mount_flags() altogether.

(This drops the statvfs() fallback that get_mount_flags() did. That
fallback was incomplete however, and mostly hid errors. Our primary
avenue to get mount flags is /proc/self/mountinfo and we should trust
it, and fix bugs we might encounter with it, but not tape over it.
Dropping the fallback is relevant in particular as it actually returned
mount flags for any path, not just mount points, which was very icky.)