Previously we looked for the specific tagged snapshot, but that doesn't
actually work properly. Now simply find the most recent snapshot on dest
and compare to src. It must exist on src.
Execution should not continue upon failure of certain zfs commands. In
particular, cleanup/rotation of the oldest snapshots will no longer
occur upon a failure to send/receive.
Change zfs receive command to use -e (was -d).
Allow greater control of dest placement. srcpool/a/really/long/path
should not force me to deal with destpool/backupdest/a/really/long/path.
* supports either local or remote usage
* always uses sudo
* getopts for a few settings
* keeps its own snapshots for send/recv purposes, independent of others
