need to access this when we have the proclist locked for reading,
and thus cannot store it in the PCB (which may be swapped out).
As part of this, call pmap_activate() from cpu_switch() to switch
to the new address space, and refresh the PCB's copy of the LDT
selector from the pmap structure (see above paragraph). We need
to do this for MP support anyhow.
Fixes a "panic: spinlock_switchcheck: CPU 0 has 1 spin locks" via
gdt_compact() reported by Nathan Williams.
which set the LDT and share VM space (e.g. new versions of WINE) expect
the LDT to be logically coupled to the address space. Use the new pmap_fork()
interface to copy non-shared user-set LDTs when the address space is forked.
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.
