Use "," as sed delimiter instead of "/"

The postinst script of libpam_yubikey will generate an error when module_args contains a "/":
"sed: -e expression #1, char 76: unknown option to `s'"
module_args can contain a "/" in the base64 encoded API key. E.g. "mode=client try_first_pass id=12345 key=abcdef0123456789/+aaaaaaaaa=". Using a different delimiter that does not occur in the replacement solves the problem.