[lastfm/jsonapi/ws/main] New JSON-API-endpoints for Last.fm integration

In preparation for the new endpoints the following refactorings where
done in lastfm.c:
- Initialize lastfm.c in main.c instead of on first scrobble attempt
(necessary to retrieve the correct lastfm status)
- Return error messages if login attempt failed
- Add api function for status and logout
- Notify about Last.fm status changes

Unrelated changes in lastfm.c are:
- Refactoring of request_post
- Add request parameters sorted alphabetically and remove calling the
sort method in request_post

src/main.c

@@ -70,6 +70,9 @@ GCRY_THREAD_OPTION_PTHREAD_IMPL;
 #include "player.h"
 #include "worker.h"
 #include "library.h"
+#ifdef LASTFM
+# include "lastfm.h"
+#endif
 
 #ifdef HAVE_LIBCURL
 # include <curl/curl.h>
@@ -784,6 +787,10 @@ main(int argc, char **argv)
   mdns_no_mpd = 1;
 #endif
 
+#ifdef LASTFM
+  lastfm_init();
+#endif
+
   /* Start Remote pairing service */
   ret = remote_pairing_init();
   if (ret != 0)