not in use, so we can always use that. Thus, when the
scheduler switches threads, it also changes the TSS's
stack pointer to point to the new thread's kernel stack.
- (The call is in schedule_tail() in thread.c.)
+ (The call is in thread_schedule_tail() in thread.c.)
See [IA32-v3a] 6.2.1 "Task-State Segment (TSS)" for a
description of the TSS. See [IA32-v3a] 5.12.1 "Exception- or