Make audio(9) get_props() optional, remove it from duplex drivers

Make audio(9) get_props() optional, remove it from duplex drivers

The property bits of audio(9) are obsolete and ought to be removed
completely.

sys/dev/audio.c:audio_open() currently uses get_props() to bail out if
read *and* write was requested on a non-duplex driver.

Drivers that currently support playing but not recording need adjustment
before the API can be cleaned up.

Drivers that advertise themselves as full duplex, i.e. those that always
return AUDIO_PROP_FULLDUPLEX unconditionally in their get_props() currently
always succeed this check.

As this is the only property, losen audio_open()'s DIAGNOSTIC check and only
do the duplex check if the driver provides get_props().

This allows for simple removal of get_props() from full-duplex drivers
without adding any other code or without changing functionality.

This includes all audio drivers under sys/dev/pci/ (maestro(4) being the
only unfinished exception here).

Other drivers as well as the API change can then follow in smaller diffs.

This builds on amd64, arm64, i386, macppc and sparc64.
amd64 with azalia(4) still plays, records as well as plays and records
at the same time on my X230 as tested with

        $ aucat -i play.wav [-o rec.wav]

alpha and hppa tests by miod
OK ratchov miod