This change introduces a new jail command hook that runs before any other action.
Such a 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.
I also took the liberty of reordering the IP_* enum, to collect exec options (sorted alphabetically) separately from exec command (sorted chronologically).