[spotify] Reintroduce libspotify support, change spotifyc to librespot-c

Select use of either libspotify or librespot-c as streaming backend via config
option.

librespot-c (renamed/improved spotifyc) impl has the following:
- sync interface
- seek support
- honor bitrate config, set client and thread name
- use web access token with "streaming" scope for login
- fix issue with podcast playback

Also say goodbye to file-based Spotify login.

src/input.h

@@ -16,9 +16,12 @@ enum input_types
   INPUT_TYPE_HTTP,
   INPUT_TYPE_PIPE,
   INPUT_TYPE_TIMER,
-#ifdef SPOTIFY
+#ifdef SPOTIFY_LIBRESPOTC
   INPUT_TYPE_SPOTIFY,
 #endif
+#ifdef SPOTIFY_LIBSPOTIFY
+  INPUT_TYPE_LIBSPOTIFY,
+#endif
 };
 
 enum input_flags