MEDIUM: task: make __tasklet_wakeup_on() only accept non-local threads

MEDIUM: task: make __tasklet_wakeup_on() only accept non-local threads

The ambiguity in usage for __tasklet_wakeup_on() is now gone. All known
callers that used to be able to pass a negative value now call
__tasklet_wakeup_here(), and remaining ones always pass an explicit
thread number. This means that we can remove the "if (thr<0)" branch,
but still leave a BUG_ON_HOT() to catch any possibly missed case. The
comment around tasklet_wakeup_on() not supporting remotely waking a
tasklet whose tid<0 was also removed since it was addressed long ago.