[kernel] Use full protected mode entry when enabling unreal mode #2308
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When XMS is enabled, this PR adds by default a full protected mode entry/exit in order to set the segment cache registers, necessary on some 386 systems for XMS memory access for system buffers and the XMS ramdisk in the kernel.
Should a system not boot with XMS enabled, this can be turned off by setting USE_PROTMODE=0 in elks/arch/x86/lib/unreal.S, or by setting xms=off or leaving out any xms= line at all.
The method being used is based on @Mellvik's code, from his research and debugging on real hardware and extensively discussed in Mellvik/TLVC#161 and before that in Mellvik/TLVC#160 (comment).
The documentation of the definition of the Global Descriptor Table has been updated as well, since it was found incorrectly describing the requirement for code versus data segments in the GDT Access Descriptor Byte.