audio/portaudio: Add support for enumerating multiple sndio devices

sndio has no device discovery mechanism and PortAudio has no way
of accepting raw device strings from users. Normally we just expose
the default device (which can be changed via the AUDIODEVICE
environment variable). This however means that we are stuck with
only one device at a time for playback/recording and would have to
restart applications (or the sndiod instance) to switch between
devices.

This is detrimental to the workflow in applications like Audacity
and makes them unusable in sndio only setups.

Add a new PA_SNDIO_AUDIODEVICES environment variables with which
users can specify and expose a list of 16 additional sndio devices
to applications using PortAudio.

Example:
PA_SNDIO_AUDIODEVICES=snd/0.monitor:snd@remote/0:rsnd/3