Improve logic for determining legacy mode

Now it will also check if the old config location exists and if it
doesn't then the new config mode will be used by default. This should
fix #858.

Author: asarium

code/osapi/osapi.cpp

@@ -19,9 +19,7 @@
 #include "cmdline/cmdline.h"
 #include "osapi/osapi.h"
 
-
-#include <SDL_assert.h>
-
+#include <fstream>
 #include <algorithm>
 
 namespace
@@ -304,6 +302,11 @@ void os_sleep(uint ms)
 #endif
 }
 
+static bool file_exists(const SCP_string& path) {
+	std::ofstream str(path, std::ios::in);
+	return str.good();
+}
+
 bool os_is_legacy_mode()
 {
 	// Make this check a little faster by caching the result
@@ -318,19 +321,31 @@ bool os_is_legacy_mode()
 		checkedLegacyMode = true;
 	}
 	else {
+		bool old_config_exists = false;
+		bool new_config_exists = false;
+
 		SCP_stringstream path_stream;
 		path_stream << getPreferencesPath() << DIR_SEPARATOR_CHAR << Osreg_config_file_name;
 
-		// Use the existance of the fs2_open.ini file for determining if the launcher supports the new mode
-		auto file = fopen(path_stream.str().c_str(), "r");
+		new_config_exists = file_exists(path_stream.str());
+#ifdef SCP_UNIX
+        path_stream.str("");
+		path_stream << Cfile_user_dir_legacy << DIR_SEPARATOR_CHAR << Osreg_config_file_name;
 
-		if (file == nullptr)
-		{
+		old_config_exists = file_exists(path_stream.str());
+#else
+		// At this point we can't determine if the old config exists so just assume that it does
+		old_config_exists = true;
+#endif
+
+		if (new_config_exists) {
+			// If the new config exists then we never use the lagacy mode
+			legacyMode = false;
+		} else if (old_config_exists) {
+			// Old config exists but new doesn't -> use legacy mode
 			legacyMode = true;
-		}
-		else
-		{
-			fclose(file);
+		} else {
+			// Neither old nor new config exists -> this is a new install
 			legacyMode = false;
 		}
 	}