Merge branch 'refs/heads/t/prune'

A couple of cleanups and unifications for the snapshot pruning code,
which is executed by the prune and run subcommands. With the patches
applied, both subcommands behave identically, i.e. they consider
the same set of snapshots as candidates for pruning. Also the prune
command gained the new --disk-space option to force it to act as if
disk space was high or low.

Cooking for six weeks.

* refs/heads/t/prune:
  Introduce prune --disk-space.
  Revamp com_prune().
  Factor out find_removable_snapshot().
  prune Simplify rm exit code logic.
  find_oldest_removable_snapshot(): Improve log message.
  prune: Print a message if there is nothing to prune.
  get_snapshot_list(): Add comment about sorting.
  prune: Fail gracefully if pre-rm hook fails.

diff --cc dss.c
Simple merge