Windows: Use backwards compatible registry key for default cache

On Windows, existing applications determine and change the default
credentials cache using the Kerberos for Windows registry key
(\Software\MIT\Kerberos5, ccname).  Use it for backwards
compatibility.

lib/krb5/cache.c

@@ -512,6 +512,12 @@ krb5_cc_set_default_name(krb5_context context, const char *name)
 		context->default_cc_name_env = strdup(e);
 	    }
 	}
+
+#ifdef _WIN32
+        if (e == NULL) {
+            p = e = _krb5_get_default_mit_cc_name();
+        }
+#endif
 	if (e == NULL) {
 	    e = krb5_config_get_string(context, NULL, "libdefaults",
 				       "default_cc_name", NULL);