Provide null segment selector as associated constant on SegmentSelector

[prinzdezibel]

This may be useful when used as a segment selector to load a non-existent segment into the data register.
e.g.

DS::set_reg(gdt.null_segment_selector())

[josephlr]

I think this method might be useful, but it doesn't seem like it needs to be a method on our GDT. It doesn't read any data from the GDT after all.

I think an associated constant like:

pub struct SegmentSelector(pub u16);

impl SegmentSelector {
    /// Some comment explaining why we want this
    pub const NULL: Self = Self::new(0, PrivilegeLevel::Ring0);

    ...
}

would be a better idea.

[prinzdezibel]

@josephlr

It doesn't read any data from the GDT after all.

Correct, I was thinking the same. The reason I initially thought it may make sense nonetheless was the fact that the GDT already has a null descriptor inside (which is not read - as you've noticed correctly - because it's easier to construct it as SegmentSelector directly instead of building it from the bytes of the null descriptor)
Also the concept that the SegmentSelector knows about the index of the descriptor in the GDT led me to think that this might be a good idea.

But I agree that a dedicated SegmentSelector might even be a better idea. I'm going to adapt my MR. Thanks.

[Freax13]

Looks good to me, thanks!