Minor ALSA tweaks (include-once macro name, len for memset() more clear).

--HG--
extra : rebase_source : 44c8b456ce5867d127b8e307c7854d1bab882a50
Disk audio target was using this->hidden->mixlen before we set it.

--HG--
extra : rebase_source : 0ca3a2c97a59010e12df6a144757b6cadb4232c5
Added an SDL2 OpenBSD sndio(7) audio target.

--HG--
extra : rebase_source : 5ad387265cff73c1635ca4f2c3635848ba722614