Allow for either px or ix dolphin in builds

Models and cdsrfm need dolphin symvers. The code needs to look for either ix or px dolphin symvers.

We are assuming we would never have both installed, but in that case, the goal is to have px files overwrite ix files.

find_module_symvers has been changed to look for multiple module names. It will return all symvers found among all the models.

Closes #422 (closed).