pool: Allow mover queues with zero capacity

PoolV4#mover_set_max_active allows the queue capacity to be set
to zero, yet the AdjustableSemaphore rejects this value (even
though the default capacity for a newly created semaphore is
zero).

Target: trunk
Require-book: no
Require-notes: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5425/