Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

drivers/alsa/audio_driver_alsa.cpp

@@ -331,14 +331,4 @@ void AudioDriverALSA::finish() {
 	finish_device();
 }
 
-AudioDriverALSA::AudioDriverALSA() :
-		thread(nullptr),
-		pcm_handle(nullptr),
-		device_name("Default"),
-		new_device("Default") {
-}
-
-AudioDriverALSA::~AudioDriverALSA() {
-}
-
-#endif
+#endif // ALSA_ENABLED