Validate that midi and audio device numbers are integers.

fix spacing

Prefix by '_' symbols that are not part of the API.

ok deraadt, guenther

Add a "device number" component in sndio(7) device names, allowing a
single aucat instance to handle all audio and MIDI services. Since
this partially breaks compatibility, this is a opportunitiy to fix few
other design mistakes (eg ':' being used by inet6, type name vs api
name confusion, etc..). This leads to the following names:

type[@hostname][,unit]/devnum[.option]

The device number is the minor device number for direct hardware
access (ie the 'N' in /dev/audioN). For aucat, this is the occurence
number of the -f (or -M) option.

There's a compatibility hook to keep old names working if only one
aucat server is running.

Make aucat audio and midi backends share the same code to communicate
with the server. As we're at it use the same protocol for midi and
audio. Now, both audio and midi code use the same SNDIO_DEBUG environment
variable to turn on/off DPRINTF's.