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Fix #13. I've duplicated the code used to create obj/k-firstprocess.h (and I'm not sure how much of that was necessary).

Set register %bl rather than %dx as instructed in https://wiki.osdev.org/X86-64 and all the sample code I could find.

Correct the names of the disk images in README.md and reformat how the disk images are created to highlight their similarities and differences.

The previous sys_panic() output tended to scroll important stuff
off the top of the screen.