audiod: Fix memory leak in writer setup.

audiod: Fix memory leak in writer setup.

If no writers are given, the default writer is used for each supported
audio format. In this case, we called the writer's command line parser
on each open, rather than once at startup as we fo for the non-default
case. This resulted in a memory leak which is fixed in this patch by
setting up the default writer only once, in the same way the default
receivers and filters are set up.