Various output fixes:

- Fixed several possible segfaults when selecting non-existing tracks in HLS
- Fixed error when requesting a stream with no compatible HLS tracks
- Fix a possible segfault when playing back an invalid track
- Fixed HLS cache control logic
- Fixed slow lookup of blank hostnames (now skips lookup entirely)
- HLS output replies with 204 rather than 200 for OPTIONS requests
- Removed out-of-the-box compatibility with iOS 10 and lower in the HTML output, in favor of better iOS 11+ support.

lib/socket.cpp

@@ -242,6 +242,7 @@ void Socket::hostBytesToStr(const char *bytes, size_t len, std::string &target){
 /// Returns empty string if no reasonable match could be made.
 std::string Socket::resolveHostToBestExternalAddrGuess(const std::string &host, int family,
                                                        const std::string &hint){
+  if (!host.size()){return "";}
   struct addrinfo *result, *rp, hints;
   std::string newaddr;