Page frame allocator

[ramidzkh]

• The PML4 can be rebuilt inside the kernel to better suit the page frame allocator. This rebuilt PML4 should
  • identity the existing running code,
  • map all regions, as provided by BIOS E820, to 0xFFFF800000000000 or some other high address,
  • isolate user code.
• The frame allocator should be able to map in arbitrary frames to pages upon request, such as the APIC.
• The frame allocator should be able to map in a free frame and yield its virtual address, for use by a dynamic allocator.
• The frame allocator should use higher-half canonical virtual addresses, if possible.