This change introduces new jail command hooks that run before and after any other actions.
The exec.prepare hook can be used for example to invoke a script that checks if the jail's root exists, creating it if it does not. Since arbitrary variables in jail.conf can be passed to the command, it can be pretty useful for templating jails.
An example use case for exec.release would be to remove the filesystem of an ephemeral jail.
The names "prepare" and "release" are borrowed from the names of similar hooks in libvirt.