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

Remove drain(), query_encoding(), mappage() and get_default_params()
methods from all audio drivers and from the audio_if structure as they
are never called.

Remove all audio format conversion code from the kernel (btw holding
the kernel_lock), as we already do better conversions in
user-mode. Yet, no need for every single driver to fiddle with the
conversion code as they are done transparently by common MI code. With
help from armani and miod, support from mpi

ok armani@

remove unused variable

Introduce a global interrupt-aware mutex protecting data
structures (including sound-card registers) from concurent
access by syscall and interrupt code-paths. Since critical
sections remain the same, calls to splraise/spllower can be
safely replaced by calls to mtx_enter/mtx_leave with two
exceptions: (1) mutexes are not reentrant (the inner splraise
is thus removed), and (2) we're not allowed to sleep with a
mutex (either msleep is used or the mutex is released before
sleeping).

ok and help from kettenis, a lot of work from armani

Add the possibility for a MD attachment to stack a stream filter (sw_code)
if it needs to rewrite the audio stream; adapted from NetBSD

Add emulation support for slinear:8, ulinear:8 and alaw. Written by jason@
more than 8 years ago; allows aucat (and sndio users) to happily use
amd7930-based audio devices.
Verified by playing an mp3 file on vax.

MI driver for AMD79C30 audio chip; from NetBSD