Use /dev/audio or pkgsrc DEVOSSAUDIO for OSS on NetBSD.

Port of two downstream pkgsrc patches.

The NetBSD-specific OSS driver was referencing /dev/sound instead of /dev/audio. The other part of this patch was already independently fixed in #53. I don't know if this actually makes much of a difference, but /dev/audio seems to be the preferred one to use. http://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/audio/xmp/patches/patch-src_drivers_netbsd.c?rev=1.4&content-type=text/x-cvsweb-markup
The generic OSS driver enables by default for NetBSD but did not use the correct device for NetBSD. I added /dev/audio to the end of the device list, as well as a compile time enabled DEVOSSAUDIO for pkgsrc at the start of the device list. http://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/audio/xmp/patches/patch-src_sound_oss.c?rev=1.1&content-type=text/x-cvsweb-markup

This fixes the generic OSS driver in my NetBSD 9.2 VM.

Combining the OSS, NetBSD, and Solaris drivers should probably at least be humored at some point, if possible.

libxmp / xmp-cli