We need to ensure that the supports task always has the results of the probing,
no matter if the probing was just launched by the plugin grabbing the port, or
by a previous plugin. We do this during supports_port(), by propagating to the
supports task any possible previously cached probing results.
This is because the cinterion plugin can handle RS232 modes, and checking
support for them needs to have the vendor ID probed with AT commands, so
probing is almost always issued in this plugin. By sorting last, we let
other plugins check support first.