audio: -audiodev command line option basic implementation

Audio drivers now get an Audiodev * as config paramters, instead of the
global audio_option structs.  There is some code in audio/audio_legacy.c
that converts the old environment variables to audiodev options (this
way backends do not have to worry about legacy options).  It also
contains a replacement of -audio-help, which prints out the equivalent
-audiodev based config of the currently specified environment variables.

Note that backends are not updated and still rely on environment
variables.

Also note that (due to moving try-poll from global to backend specific
option) currently ALSA and OSS will always try poll mode, regardless of
environment variables or -audiodev options.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: e99a7cbdac0d13512743880660b2032024703e4c.1552083282.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>