sr: add probe list to device instance

sr: add probe list to device instance

There is no point in libsigrok copying probe lists around. The driver now
builds a list of probes according to the model device it found, and will
make that available to a frontend. The frontend thus has a reference of
what the driver has, including default names, and doesn't need libsigrok
to provide an unnecessary level of abstraction.

The sr_probe_new() library-private function is a helper for drivers.