super-show: Show sync mode and cache replacement policy
super-show: Print label
udev: Fit into the standard rule sequence

Move the rule to run a bit later, after
the standard udev rules have called blkid.

Don't run blkid manually, and run probe-bcache
as a fallback which can be patched out in a
package that depends on util-linux 2.24+.

probe-bcache: bail if libblkid finds anything
Preserve our last-resort safety check here
in case earlier rules fail to call blkid.
Merge branch 'master' into debian

* master:
  udev: Fit into the standard rule sequence
  super-show: Print label
  super-show: Show sync mode and cache replacement policy