Fix #1351 - Server ignores only-reg if in "none" auth mode.

When in none auth mode, registered users don't exist; hence the setting
makes the game impossible to join.

Normally the setting would be greyed out when the user is unregistered,
but commit 475c54bf introduced a new edge case where the stored setting
would apply to the field even if the user is unregistered, making the
setting possible to apply in no-reg mode. Regardless, any client could
have done this, not just the default cockatrice.

The server side fix should prevent all issues in the future.

common/server.h

@@ -55,6 +55,7 @@ public:
     void removeClient(Server_ProtocolHandler *player);
     virtual QString getLoginMessage() const { return QString(); }
 
+    virtual bool permitUnregisteredUsers() const { return true; }
     virtual bool getGameShouldPing() const { return false; }
     virtual bool getClientIdRequired() const { return false; }
     virtual bool getRegOnlyServer() const { return false; }