Storing crypto keys to the X509 file system key store could fails due to inappropriate chars in filenames

Naming of keystore keys is based on the subject identifier encoded in base64. This is nice but base64 is not appropriate for filenames (contains slashes, and is case sensitive).
So, to avoid spurious problem while storing certain keys, I will change the naming to use Hex encoding.
This is a breaking change that will make all existing key unreadable, but since nobody has ever reported the problem, I doubt anybody is using it, so let change it now, once for all.
If you were using it, please backup your keys to a wiki store before changing, and restore them back later, after upgrading to this new storage.