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.
`for (` and `} else {` below.

Personally, I would instead use a lookup:
```c++
const char hexdigits[16] = "09123456789ABCDEF";

for (int i = 0; i < 6; ++i) {
	u8 octet = rand() % 256;
	randMAC += hexdigits[(i >> 0) & 15];
	randMAC += hexdigits[(i >> 4) & 15];
	if (i < 5) {
		randMAC += ':';
	}
}
```

Technically, I think the first octet should have bit 2 set, and bit 1 not set, technically, but probably this won't matter... I don't really know for sure.

-[Unknown]