Deduplicate --channels and --sample-rate.

Deduplicate --channels and --sample-rate.

These options are identical for all writers. Moreover, they only
make sense for para_write but are ignored for para_audiod.

So move these options from the writer-specific ggo file to write.m4,
the config file for para_write. The writers obtain the sample rate
and the channels count as before via the buffer tree exec mechanism,
but it is now the check_wav node rather than the btr node of the writer
that computes this information from the given option, the wav header
or the builtin default.

The new HANDLE_EXEC macro contains the logic for determining the
source of both the sample rate and the channels count.