[raop] Add support for Apple TV device verification, required by tvOS 10.2 (fix for issue #377)

- also change how speakers are saved/retrieved from the db
- add generic authorization methods in outputs.c and player.c
- let filescanner read *.verification files (containing PIN)
- configure options to enable and disable, since libsodium is required

src/main.c

@@ -606,6 +606,9 @@ main(int argc, char **argv)
 #ifdef MPD
   strcat(buildopts, " --enable-mpd");
 #endif
+#ifdef RAOP_VERIFICATION
+  strcat(buildopts, " --enable-verification");
+#endif
 #ifdef HAVE_ALSA
   strcat(buildopts, " --with-alsa");
 #endif