Extract StcAutoProtocol class, fix autodetection

With the introduction of real abstract classes, it is not possible
anymore to instantiate StcBaseProtocol. Instead, extract some of the
code for autodetection into the new class StcAutoProtocol and use
that for autodetection.

test/test_program.py

@@ -133,4 +133,3 @@ class ProgramTests(unittest.TestCase):
             read_mock.side_effect = convert_to_bytes(test_data["responses"])
             gal = stcgal.frontend.StcGal(opts)
             self.assertEqual(gal.run(), 0)
-