super-show: make the version annotation more terse
super-show: show more cache-related information

Only show nbuckets and nr_this_dev on cache devices;
explain nbuckets' relation to device size by printing
both total_sectors (superblock included) and
cache_sectors (superblock excluded).
super-show: don't show cache_count

This is a cache-set property, but not terribly useful.
Showing it on backing devices was also incorrect.
super-show: print cache mode (writethrough, writeback, writearound, none)
super-show: show detached/clean/dirty/inconsistent status