[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/inputs/libspotify/libspotify.h

@@ -0,0 +1,65 @@
+
+#ifndef __LIBSPOTIFY_H__
+#define __LIBSPOTIFY_H__
+
+#include <event2/event.h>
+#include <event2/buffer.h>
+#include <event2/http.h>
+#include <stdbool.h>
+
+
+struct spotify_status_info
+{
+  bool libspotify_installed;
+  bool libspotify_logged_in;
+  char libspotify_user[100];
+};
+
+#define LIBSPOTIFY_SETUP_ERROR_IS_LOADING -2
+
+int
+libspotify_playback_setup(const char *path);
+
+int
+libspotify_playback_play(void);
+
+int
+libspotify_playback_pause(void);
+
+//void
+//spotify_playback_pause_nonblock(void);
+
+int
+libspotify_playback_stop(void);
+
+//void
+//spotify_playback_stop_nonblock(void);
+
+int
+libspotify_playback_seek(int ms);
+
+//int
+//spotify_artwork_get(struct evbuffer *evbuf, char *path, int max_w, int max_h);
+
+int
+libspotify_relogin(void);
+
+int
+libspotify_login(const char *user, const char *password, const char **errmsg);
+
+void
+libspotify_logout(void);
+
+void
+libspotify_status_info_get(struct spotify_status_info *info);
+
+void
+libspotify_uri_register(const char *uri);
+
+int
+libspotify_init(void);
+
+void
+libspotify_deinit(void);
+
+#endif /* !__LIBSPOTIFY_H__ */