MEDIUM: task: only keep task_new_*() and drop task_new()

MEDIUM: task: only keep task_new_*() and drop task_new()

As previously advertised in comments, the mask-based task_new() is now
gone. The low-level function now is task_new_on() which takes a thread
number or a negative value for "any thread", which is turned to zero
for thread-less builds since there's no shared WQ in thiscase. The
task_new_here() and task_new_anywhere() functions were adjusted
accordingly.