Go to protected mode

Use Makefile.build and build prettily. Cleaner project tree with
sub-directories.

The GDT was BS. This (obviously) triple-faulted the kernel so that qemu went
power-cycling. However, we may not (as I conclude from various source code)
re-enable interrupts once we are in protected mode but don't have an IDT set
up. It seems that a GPF is fired immediately then which, again,
triple-faults. I don't know the reason for the immediate GPF but the rest
sounds logical.

I certainly have to do the next steps with Tanenbaum and Intel Manuals...

The rest, i.e. src/, is bogus. I want to write major parts in C *later*.