Update GDT code in post (no data segments anymore)
Update GDT code in post (no data segments anymore)

We also load 0 into all data segment registers to avoid future problems with iretq. Also: minor wording improvements.

phil-opp/blog_os,phil-opp/blogOS,phil-opp/blog_os,phil-opp/blog_os,phil-opp/blog_os,phil-opp/blogOS