MINOR: task: add a new explicitly local tasklet wakeup function

MINOR: task: add a new explicitly local tasklet wakeup function

The current tasklet_wakeup() call relies on tasklet_wakeup_on(tl->tid),
which was already quite ambiguous till now due to the sole reliance on
tid being negative or not to decide to run locally, but it no longer
works correctly if used to wake tasks up since the new set of possible
negative values for ->tid (particularly if some code calls
__tasklet_wakeup_on() on a task as is done in task_instant_wakeup()).

The problem is that it is not possible in the current API to explicitly
say that we want a task/tasklet to run locally or remotely without having
to play games with a thread number. The chosen approach to address this
is to change tasklet_wakeup_on() to always be remote and have
tasklet_wakeup_here() which will always be local, with tasklet_wakeup()
choosing one or the other depending on the tid, for backwards compat
only.

This patch implements tasklet_wakeup_here() to __tasklet_wakeup_here()
that reimplement the part of __tasklet_wakeup_on() that used to deal
with the local thread only (negative tid). No other change was made.
For now it remains unused.

The doc was updated.