Windows: Registry based configuration

Load configuration data in the registry into a krb5_config_section.
Each registry key corresponds to a krb5_config_section and each
registry value becomes a bound string value.

The set of values contained in the root Heimdal registry key is
treated as if they were defined in the [libdefaults] section.

E.g. the configuration file:

[libdefaults]
foo = bar

[Foo]
x = y
y = {
  baz = quux
}

is equivalent to the registry keys:

[HKEY_CURRENT_USER\Software\Heimdal]
"foo"="bar"

[HKEY_CURRENT_USER\Software\Heimdal\Foo]
"x"="y"

[HKEY_CURRENT_USER\Software\Heimdal\Foo\y]
"baz"="quux"

lib/krb5/context.c

@@ -616,6 +616,11 @@ krb5_set_config_files(krb5_context context, char **filenames)
     if(tmp == NULL)
 	return ENXIO;
 #endif
+
+#ifdef _WIN32
+    krb5_load_config_from_registry(context, &tmp);
+#endif
+
     krb5_config_file_free(context, context->cf);
     context->cf = tmp;
     ret = init_context_from_config_file(context);