Make botan’s policy configurable from a file

fix #3244

src/utils/dirname.cpp

@@ -0,0 +1,16 @@
+#include <utils/dirname.hpp>
+
+namespace utils
+{
+    std::string dirname(const std::string filename)
+    {
+      if (filename.empty())
+        return "./";
+      if (filename == ".." || filename == ".")
+        return filename;
+      auto pos = filename.rfind('/');
+      if (pos == std::string::npos)
+        return "./";
+      return filename.substr(0, pos + 1);
+    }
+}