Add protocol handler support

This lets the user click a link in a web browser to very easily join a Quake 3 multiplayer game. As browser-based matchmaking websites become more popular, this makes it a lot more convenient and simple to play Quake 3 with others.

The links have the following URI format: quake3://connect/example.com:27950. The format has been designed to be flexible to allow more types of links in the future and avoiding having to make a breaking change. At the moment, "connect" is the only supported command.

code/sys/sys_local.h

@@ -64,3 +64,8 @@ void Sys_AnsiColorPrint( const char *msg );
 
 int Sys_PID( void );
 qboolean Sys_PIDIsRunning( int pid );
+
+#ifdef PROTOCOL_HANDLER
+char *Sys_InitProtocolHandler( void );
+char *Sys_ParseProtocolUri( const char *uri );
+#endif