Map kernel stacks only at unique addresses.
Use one TSS per process.
Add sysarch calls for modifying IOPL and the I/O permission bitmap.
Add a compacting GDT entry allocator, for TSS and LDT selectors.
Enable modifying %fs and %gs with PT_SETREGS.
Sanitize various bits of code.
also, delete r_fs and r_gs; looking at the code and comments
describing/using this struct, they don't belong here
(though if support for them is ever put in here, they should be,
but for i486-en only... how do you do that 'reasonably,' though?
interface issues...)
