Feature request: disable USB on poweroff

```
Currently as soon as Squeezelite is started it will send silence (all zeros)
out the USB port. It seems it would be better to send nothing out the USB port
when squeezelite is "powered off"

The LMS web interface and iPeng etc have a power on/off button for the player.
Even though squeezelite is not a hardware player it can receive the "power off"
command.

One of my (VortexBox) forum a user has a Wavelength Proton USB DAC. This is a
battery powered DAC that recharges it battery through the USB port when it's
not in use. Because with Squeezelite the output is always "on" it will never
recharge.

```

Original issue reported on code.google.com by `and...@vortexbox.org` on 8 Apr 2014 at 1:00