As a header-only C++ library, synced OSS operation (sosso) is used to
clean up and separate the low-level handling of FreeBSD OSS devices
in JACK. Features include:

 * Supports both read() / write() and mmap() IO operation.
 * Adaptive polling, better suited for low-latency requirements.
 * Internal double buffer to avoid troubles with OSS buffer resize.
 * Closely monitors progress and drift for each channel (+/- 1ms).
 * Facilitates drift correction through buffer offsets.
 * Depends on C++ standard library and system headers, nothing else.

Although the sosso library is somewhat tailored to the needs of JACK,
it was developed separately and will eventually be published and / or
used in other projects. Therefore the headers follow a different
formatting style and are more liberally licensed (ISC).

Reduce the base latencies for capture and playback by half a period,
and let the update method account for the additional playback latency
introduced by OSS buffer management.
This fits actual OSS latencies better, so the same settings for the
extra input-latency and output-latency parameters should apply to
different period lengths.

Beware that this change invalidates current input-latency and
output-latency values, they have to be measured again.

Fixes a scan-build warning about using uninitialized values.

While there, also improve the short description of this option in the
OSS backend part of usage help and jack_control.

Features:
 * Internal workings more in line with other Jack drivers.
 * Use poll() for wait and sync instead of blocking I/O.
 * Allows to use Jack in "async" mode.
 * Calculate DSP usage correctly.
 * OSS buffer management to achieve stable latencies.
 * Latency correction for asymmetric OSS buffer use.
 * More robust handling of over- and underruns.
 * Handle format changes forced by the OSS interface.
 * FreeBSD 24bit samples are always packed.

No changes to the driver parameters or the user interface.

Start with an unchanged copy of the Solaris OSS driver.