[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/misc.c

@@ -70,6 +70,12 @@ static char *buildopts[] =
 #else
     "Without Spotify",
 #endif
+#ifdef SPOTIFY_LIBRESPOTC
+    "librespot-c",
+#endif
+#ifdef SPOTIFY_LIBSPOTIFY
+    "libspotify",
+#endif
 #ifdef LASTFM
     "LastFM",
 #else