Importing aiomixer to src

[nia <nia%NetBSD.org@localhost>]

For those unfamiliar, aiomixer is my name for my graphical
(curses-based) frontend to NetBSD mixer devices.

It has similar functionality to mixerctl, but is designed to be more
user-friendly for interactive use. For example, hold the arrow keys,
the system's volume changes, you get instant visual feedback.
Like a real hardware mixer! Except ASCII and without twisty knobs.

Recently I rewrote it to (a) fix the handling of SET mixer controls
(b) remove the CDK/ncurses dependencies (the only dependency is
now NetBSD libcurses). The binary is 13K stripped and compiles with
WARNS=6.

I think this version is suitable for importing into NetBSD base
for continued development. This will also allow us to easily
keep it in sync if there are any changes to the audio stack.

Review and comments welcome, of course.

https://github.com/niacat/aiomixer2