BUG/MEDIUM: task: Don't free a task that is about to be run.

BUG/MEDIUM: task: Don't free a task that is about to be run.

While running a task, we may try to delete and free a task that is about to
be run, because it's part of the local tasks list, or because rq_next points
to it.
So flag any task that is in the local tasks list to be deleted, instead of
run, by setting t->process to NULL, and re-make rq_next a global,
thread-local variable, that is modified if we attempt to delete that task.

Many thanks to PiBa-NL for reporting this and analysing the problem.

This should be backported to 1.8.