Large update to warden backend.

1. Start by updating various commands, remove {zfs} prefix from them

2. Create a new "snap" sub-command, which will handle aspects of
   snapshot management, such as listing, creation, removal, reverting

3. Rewrite our "clone" functionality. Instead of just cloning to a
   new directory for the user to inspect, now you can run:

   # warden clone myjail newjail --ipv4=127.0.0.9/24

   This command will take an instant snapshot of "myjail" and clone it
   to the new jail "newjail", setting the new IP address optionally

4. Remove some legacy cloning cruft, and re-do help pages for all
   the effected commands

These changes mean that you can now create a master jail, and manually
or scripted create as many new "clones" of that jail as you like.