MINOR: tasklet: set TASK_WOKEN_OTHER on tasklets by default

MINOR: tasklet: set TASK_WOKEN_OTHER on tasklets by default

Now when tasklets are woken up via tasklet_wakeup(), tasklet_wakeup_on()
or tasklet_wakeup_after(), either the optional wakeup flags will be used,
or TASK_WOKEN_OTHER will be used.

This allows tasklet handlers waking up for any given cause to notice
whether or not they were also woken for another reason. For example, a
mux handler could skip heavy parts when seeing that TASK_WOKEN_OTHER is
absent, proving that no standard tasklet_wakeup() was done, for example
in response to a subscribe().

The benefit of the TASK_WOKEN_* flags is that they're purged during the
wakeup, and that they're easy to check for using TASK_WOKEN_ANY.
TASK_F_UEVT1 and TASK_F_UEVT2 are also usable for private use (e.g. wakeup
from a stream to a connection inside a mux).

Probably that in the future, code dealing with subscribe events should
start to place TASK_WOKEN_IO like is done for upper layers.