MAJOR: task: use t->tid instead of ffsl(t->thread_mask) to take the thread ID

MAJOR: task: use t->tid instead of ffsl(t->thread_mask) to take the thread ID

At several places we need to figure the ID of the first thread allowed
to run a task. Till now this was performed using my_ffsl(t->thread_mask)
but since we now have the thread ID stored into the task, let's use it
instead. This is tagged major because it starts to assume that tid<0 is
strictly equivalent to atleast2(thread_mask), and that as such, among
the allowed threads are the current one.