Silence compiler warning 'unused parameter running_handles' in function...

Silence compiler warning 'unused parameter running_handles' in function multi_runsingle(). This is done here returning multi->num_alive in the running_handles parameter even when functions that call multi_runsingle() at this moment overwrite the returned value with the one that is valid when those functions curl_multi_perform() and multi_socket() have removed expired timers from the splay. Most probably, parameter 'running_handles' in function multi_runsingle() should be just removed.