[sheepdog] [PATCH 1/4] sheep: get the free disk space and store it in the config file

[Christoph Hellwig]

Sorry for beeing so late to the game, but what is the benefit of storing
it?  If we'd update the free space regularly a big advtange would be
that sheepdog could deal with the fact that freespace changes.  This
can already happen just due to using sheepdog alone (e.g. log files
growing like crazy if the debug level is high), or also be useful
if the filesystem is shared with other users.