Added 'set interfaces wireguard wgX threaded' command.
Process traffic from each peer in a dedicated thread.

Using threaded as CLI node is a very deep term used by kernel threads. To make
this more understandable to users, rename the node to per-client-thread.

It's also not necessary to test if any one peer is configured and probing if
the option is set. There is a base test which requires at least one peer
to be configured.

Provides a per-device control to enable/disable the threaded mode for all the
napi instances of the given network device, without the need for a device
up/down.