Uthernet: fix usability and settings (PR #947)

* Make Uthernet settings behave like all other cards.
Fix as well the fact that the Uthernet interface would not be reapplied after a restart.

* Uthernet: use consistent types to reduce code complexity.
Use std::string everywhere.

source/Utilities.cpp

@@ -300,7 +300,7 @@ void LoadConfiguration(void)
 	tfe_enabled = dwTfeEnabled ? 1 : 0;
 
 	RegLoadString(TEXT(REG_CONFIG), TEXT(REGVALUE_UTHERNET_INTERFACE), 1, szFilename, MAX_PATH, TEXT(""));
-	update_tfe_interface(szFilename, NULL);
+	update_tfe_interface(szFilename);
 
 	//