Kernel: Unwind kernel stacks before dying

While executing in the kernel, a thread can acquire various resources
that need cleanup, such as locks and references to RefCounted objects.
This cleanup normally happens on the exit path, such as in destructors
for various RAII guards. But we weren't calling those exit paths when
killing threads that have been executing in the kernel, such as threads
blocked on reading or sleeping, thus causing leaks.

This commit changes how killing threads works. Now, instead of killing
a thread directly, one is supposed to call thread->set_should_die(),
which will unblock it and make it unwind the stack if it is blocked
in the kernel. Then, just before returning to the userspace, the thread
will automatically die.

Kernel/Syscall.cpp

@@ -100,4 +100,7 @@ void syscall_trap_entry(RegisterDump regs)
     if (auto* tracer = process.tracer())
         tracer->did_syscall(function, arg1, arg2, arg3, regs.eax);
     process.big_lock().unlock();
+
+    // Check if we're supposed to return to userspace or just die.
+    current->die_if_needed();
 }