Replace check_wav_task by write_task.

Besides the task(s) associated with the given writer(s), para_write
currently sets up two other tasks: The stdin task and the check_wav
task.

The resample filter, which is to be introduced in subsequent patches,
also needs the functionality that is provided by the check wav task.
However, as filters are not supposed to create their own tasks, there
should not be a dedicated task instance for checking the wav header.

To overcome this problem we move the task member from
check_wav.c to write.c and rename the check_wav_task struct to
check_wav_context. Hence the ->pre_select and ->post_select methods
become part of para_write that call into check_wav.c to do the work.

The patch is quite large but large parts of the changes are just
trivial cwt -> cwc conversions.