Move wav detection code to a separate file.

Move wav detection code to a separate file.

The logic to detect a wav header  and delete it from an input stream
is useful also for the upcoming resample filter, so let's make this
code reusable.

The first step for doing so is to remove the relevant parts from
write.c to check_wav.c and exports those functions which are called
from write.c to check_wav.h.

There is one problem though: The wav detector code needs to know
the arguments for the --channels, --sample_rate and --sample_format
options given to para_write, but the code would not be reusable if
check_wav.c depended on the gengetop args info struct of para_write.

Hence we introduce the public structure wav_params in check_wav.h
for this information, which is independent of gengetopt. A simple
convenience helper for copying the command line arguments from an
(arbitrary) args info structure to a struct wav_params is provided
as well.