ao_oss: don't enable --softvol with OSSv4

On FreeBSD >= 8.0 volume is per-application but, unlike 4Front OSSv4,
after reinit the old volume is lost due to hw.snd.vpc_autoreset=1 by
default. Adding no_persistent_volume=true only helps to maintain
volume between files, not when seeking. One way to solve this is to
avoid unnecessarily closing audio_fd.

Based on http://people.freebsd.org/~ariff/ports/multimedia_mplayer/patch-xzz
ao_oss: don't reset audio_fd on pause/resume

Based on http://people.freebsd.org/~ariff/ports/multimedia_mplayer/patch-xzz