Destroy cleanup

While this PR doesn't rely on the changes made in my other PRs sent today, the changes are in close proximity, so they are included to make merging easier. Please review this PR last.

This PR makes the delete logic more linear. zpool28fix is fully dropped, and  RmZfsSnapshot no longer accepts the zopt '-r'. The '-r' and '-R' flags are restored, but not to be passed directly to zfs_cmd. Instead, we mimic the functionality.

Also, $rm_snapshot_pattern was dropped. Currently, this is less efficient, but it does make it more clear, and all deletion goes through the same path now. Caching (perhaps through an eval array) could help performance if needed.

Fixes #3