make-bcache: prevent stray superblocks

Hello,
Here is a patch updating make-bcache to disallow stray superblocks, so that it matches the stricter udev check.

- https://github.com/g2p/bcache-tools/commits
- https://github.com/g2p/bcache-tools/commit/master

From the commit message:
Fail if the device has any leftover superblock or partition table label.

Add a --wipe-bcache flag to overwrite bcache superblocks and recommend wipefs for non-bcache superblocks.

Once the device is safe to write, always zero immediately before the bcache superblock.

Make sure errors go to stderr. Use pkg-config.