ports using existing file descriptors.

okay ratchov@

suggested by deraadt@

as NULL has. This will (hopefully) simplify ports where the user
passes the device string.

suggested by deraadt@

is available for the stream. As we're at it, remove macros and
functions that are neither used nor documented.

with open(2) and write(2) syscalls.
from Thomas Pfaff, jakemsr is fine with it

cannot cooperate with other programs.  The aim of this change is
to allow any program to send midi data to other programs as they
were midi hardware. For instance, this change should solve the
longstanding problem of using a midi sequencer with software
synthesizers. More precisely:
- new midicat(1) utility (actually hardlink to aucat(1)).
it creates software midi thru boxes, allowing programs
to send midi messages to other programs as they were
midi(4) hardware.
- new midi api in libsndio (see mio_open(3)), to access
midi(4) devices and midicat(1) sockets in a uniform way.
- new device naming scheme <service>:<unit>[.<option>],
common to audio and midi.
- new sndio(7) manual describing concepts and naming
The current audio device naming still works, but people having
scripts or configuration files containing device names could read
the sndio(7) man page and slowly start updating device names.
discussed with jakemsr@ and deraadt@, help form jmc@

files leave.
requested by deraadt@, ok miod@