Implement a simple native VM86 backend for X86BIOS. Now i386 uses native

VM86 calls instead of the real mode emulator as a backend.  VM86 has been
proven reliable for very long time and it is actually few times faster than
emulation.  Increase maximum number of page table entries per VM86 context
from 3 to 8 pages.  It was (ridiculously) low and insufficient for new VM86
backend, which shares one context globally.  Slighly rearrange and clean up
the emulator backend to accommodate new code.  The only visible change here
is stack size, which is decreased from 64K to 4K bytes to sync. with VM86.
Actually, it seems there is no need for big stack in real mode.

MFC after:	1 month

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