libsndfile can be used to open a UNIX-style pipe

In specific, it can be used to read in data almost directly from Gqrx
Currently, this will not function correctly using -i /dev/stdin on OS X
because stdin is not seekable, and it is not actually a file; also, for
ongoing raw data, there are no hints for libsndfile to identify the
sample.

So, we can use the UNIX-y '-' to do libsndfile-approved fd handle open.

Tested with the dsd-samples wav after converting it to correct sampletype
(stereo) -> sox p25_raw_unencrypted_edf.wav -c 2 ~/gqrx_last_recorded_filename.wav
.. and then pressing 'play' in the Gqrx interface while running:

socat udp-listen:7355 - | dsd -i - -o pa:2

.. in the background. And it decoded it nicely as many times as I played it,
which was 5 or 10.