Add Random MAC address option

Add an option for the user to randomize his or her MAC address.
I would think it would make it easier for users who don't know their MAC addresses and need a unique MAC address (for example, when connecting to an ad-hoc server host). The user's chance of having a MAC address that matches and clashes with one randomly generated is 1/16777216, so we have a nice probability that it won't clash.

The original idea was to have the default value for the address be random, and have the user specify their own if they needed a specific one, as that would minimize issues, but it could cause potential issues for users who expected the default MAC.