Guard the lower 1MB of memory

[jasoncouture]

This also adds a new fuction to ignore the attempts to guard the lower 1MB if it's really required.

Closes #311

[jasoncouture]

@phil-opp this is ready for review. I also added tests.

[phil-opp]

Thanks for the updates! I left some suggestions, otherwise these changes look good to me. However, I still think that we need to ensure that we don't use the first megabyte for the kernel allocation in the UEFI loader, as I noted above:

I think the main issue is that the UEFI allocation used for loading the kernel might already use the first MB if we are unlucky. So to be safe, we would need to allocate that first megabyte before creating the allocation. We can use the "allocate at address" API mentioned in https://github.com/rust-osdev/bootloader/issues/311#issuecomment-1372193672 for that.

[phil-opp]

@jasoncouture Friendly ping :). What's the status of this?

[jasoncouture]

@jasoncouture Friendly ping :). What's the status of this?

I'll take another look weekend after next. Got some personal things to attend for the next week or so.

[Freax13]

Superseded by #446