Restructured / tidied a bit:

* Capability.grabReturnCapability() is now called by resumeThread().
  It takes care of waiting on the (Capability.c-local) condition
  variable, 'returning_worker_cond' (moved here from Schedule.c)

* If a worker notices upon entry to the Scheduler that there are
  worker threads waiting to deposit results of external calls,
  it gives up its capability by calling Capability.yieldCapability().

* Added Scheduler.waitForWork(), which takes care of blocking
  on 'thread_ready_cond' (+ 'rts_n_waiting_tasks' book-keeping).

Note: changes haven't been fully tested, due to HEAD instability.