Speed up open_current_receiver().

Speed up open_current_receiver().

To my surprise, this function showed up in the profiling info.
This is due to the fact that the old code was parsing the
FORMAT status item in audiod_pre_select() which gets called
on each iteration of the sheduler. However, this status item
gets only modified on audio file change.

So parse the status item string only once, when we receive it,
and store the current audio format number in the new field of
struct status_task.