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/qcommon/q_shared.h

@@ -40,6 +40,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
   #define CINEMATICS_LOGO		"foologo.roq"
   #define CINEMATICS_INTRO		"intro.roq"
 //  #define LEGACY_PROTOCOL	// You probably don't need this for your standalone game
+//  #define PROTOCOL_HANDLER		"foobar"
 #else
   #define PRODUCT_NAME				"ioq3"
   #define BASEGAME					"baseq3"
@@ -56,6 +57,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
   #define CINEMATICS_LOGO		"idlogo.RoQ"
   #define CINEMATICS_INTRO		"intro.RoQ"
   #define LEGACY_PROTOCOL
+  #define PROTOCOL_HANDLER		"quake3"
 #endif
 
 // Heartbeat for dpmaster protocol. You shouldn't change this unless you know what you're doing