Kernel: Various stability improvements.

- Don't cli() in Process::do_exec() unless current is execing.
  Eventually this should go away once the scheduler is less retarded
  in the face of interrupts.

- Improved memory access validation for ring0 processes.
  We now look at the kernel ELF header to determine if an access
  is appropriate. :^) It's very hackish but also kinda neat.

- Have Process::die() put the process into a new "Dying" state where
  it can still get scheduled but no signals will be dispatched.
  This way we can keep executing in die() but won't get our EIP
  hijacked by signal dispatch. The main problem here was that die()
  wanted to take various locks.

Kernel/MemoryManager.cpp

@@ -885,7 +885,6 @@ size_t Region::amount_shared() const
 
 PageDirectory::~PageDirectory()
 {
-    ASSERT_INTERRUPTS_DISABLED();
 #ifdef MM_DEBUG
     dbgprintf("MM: ~PageDirectory K%x\n", this);
 #endif