Support static MAC address + fix clone VNET jail

[tschettervictor]

This PR adds some functionality, and some fixes.

• support for static MAC VNET jails (bridged and vnet)
• fix cloning VNET jails
• simplify some sed commands when cloning jails
• allow cloning VNET jails and regenerate MAC address for cloned jail

This PR has been tested with bridged VNET, VNET, and loopback jails. Steps below

1. Create jail
2. Ensure network works
3. Clone jail
4. Ensure network works on cloned jail
5. Ensure jail.conf is updated properly on cloned jail
6. Ensure rc.conf is updated properly on cloned jail