ao_oss: add 6.1 and 7.1 speaker placement from FreeBSD

For some reason -channels 8 doesn't work with ao_oss despite mp_chmap_from_channels_alsa() should have added `fl-fr-fc-lfe-flc-frc-sl-sr` for 7.1.

```
$ mpv -no-config -ao oss -channels 8 -msglevel all=-1:ao=6 Nums_7dot1_24_48000.wav
[ao] Trying preferred audio driver 'oss'
[ao/oss] requested format: 48000 Hz, 7.1 channels, s32
[ao/oss] using '/dev/dsp' dsp device
[ao/oss] using '/dev/mixer' mixer device
[ao/oss] using 'pcm' mixer device
[ao/oss] sample format: s32
[ao/oss] using 2 channels (requested: 2)
[ao/oss] using 48000 Hz samplerate
[ao/oss] frags:  32/32  (2048 bytes/frag)  free:  65536
```

I couldn't find correct layouts in 4Front source. However, on FreeBSD they're both documented in sound(4) manpage as vchanformat values and in matrix_map.h file.

http://bxr.su/FreeBSD/sys/dev/sound/pcm/matrix_map.h#493