Apply default umask in SetupEnvironment()

This change makes all filesystem artifacts--files and directories--being
created with the default umask.

src/util/system.cpp

@@ -1360,6 +1360,11 @@ void SetupEnvironment()
     SetConsoleCP(CP_UTF8);
     SetConsoleOutputCP(CP_UTF8);
 #endif
+
+#ifndef WIN32
+    constexpr mode_t private_umask = 0077;
+    umask(private_umask);
+#endif
 }
 
 bool SetupNetworking()