Add OSS v3 backend support

[dmgk]

Some *BSD systems (FreeBSD, TrueOS, DragonflyBSD) use older OSS v3-like API, this PR add a new backend to support it. --enable-oss configure option was extended to support OSS backend selection at configure stage:

--enable-oss or --enable-oss=v4 will select OSS v4 backend --enable-oss=v3 will select the new OSS v3 backend

This PR also fixes OSS v4 build by adding two stubs

const gchar *oss_get_device();
const GList *oss_get_device_names();

to the oss4_backend.c.