sndiod: Move controls out of the device structure

sndiod: Move controls out of the device structure

Moving to a global server-wide controls list is necessary to expose
controls that are not associated to a particular device (ex. a device
selector).

The current hack to use the device-side sioctl_desc->addr variable as
client-side key can't work anymore. So, we use a unique dynamically
allocated ctl->addr key; this is much cleaner. A new "scope" enum
(with two "void *" arguments) is used to determine what the control
does control. This adds flexibility and allows to easily add new
control types that are not associated to devices.

No behavior change.