Tweak Airplay

[yubiuser]

This PR improves the Airplay handling with the shairport-sync backend. It addresses 3 issues.

1. If compiled without HAS_EXPAT=1, a warning will be thrown and the metadata handling probably won't work.
   

   snapcast/server/streamreader/airplay_stream.cpp

   Lines 70 to 76 in a3cd985

   	#ifdef HAS_EXPAT
   	createParser();
   	metadata_dirty_ = false;
   	#else
   	LOG(INFO, LOG_TAG) << "Metadata support not enabled (HAS_EXPAT not defined)"
   	<< "\n";
   	#endif

   
   There was no documentation of this flag so far, it was only listed in the commit message of fdcdf8e. This PR adds some documentation
2. shairport-sync supports Airplay 2 now. It uses port 7000. https://github.com/mikebrady/shairport-sync/blob/6240fd83707b77fe2fb2250f8136e530e36010d2/man/shairport-sync.html#L313-L314. This PR adds documentation.
3. Airplay backend sets --use-stderr. This option is deprecated. https://github.com/mikebrady/shairport-sync/blob/6240fd83707b77fe2fb2250f8136e530e36010d2/shairport.c#L327

[badaix]

Thanks, I removed the HAS_EXPAT section, because plain make support has been dropped and CMake will use Expat, except you switch it explicitly off.