New flags for default placement.

This patch introduces support for the following command-line flag:

 -dp <PLACEMENT>

This determines the default placement for tasks in worker-based
back-ends (eg LPEL or d-snet).  <PLACEMENT> can be either:

- "single"": all tasks are created on worker 0.
- "shared-rr": all tasks are distributed round-robin, regardless of
  type.
- "entity-rr": each entity has its own round-robin counter,
  and control tasks share a separate round-robin counter.
- "dsnet": use explicit distributed S-NET placement.
  (same as argument -dloc already available)