Changed naming of the mixers to get rid of umbiguity as outputs are actually going to be driven by io, not fmu

Implemented automatic lookup for the .aux.mix file if it exists

src/systemcmds/tests/test_mixer.cpp

@@ -343,7 +343,7 @@ int test_mixer(int argc, char *argv[])
 	if (argc > 2)
 		filename = argv[2];
 	else
-		filename = "/etc/mixers/FMU_quad_w.mix";
+		filename = "/etc/mixers/quad_w.main.mix";
 
 	load_mixer_file(filename, &buf[0], sizeof(buf));
 	loaded = strlen(buf);