FreeBSD: Rewrite the OSS driver using sosso library.

Make use of the sosso library to handle the low-level operation of
OSS devices on FreeBSD. Further separate this part into the
JackOSSChannel class, which should make it easier to reuse the code
for JackOSSAdapter (not planned yet).
JackOSSChannel features an additional assist thread which helps to
process the OSS device when the driver thread is busy with the JACK
dependency graph.
Combined with the improvements of the sosso library, these changes
bring JACK into the realm of real-time audio processing on FreeBSD,
for select PCI sound cards (4-6ms round trip latency).