Move character escaping function to ipautil
Functions `escape_seq` and `unescape_seq` have a generic use-case so it makes
sense to move them from `kerberos` to ipautil module so that other modules can
reuse them more readily.
https://fedorahosted.org/freeipa/ticket/5809
Reviewed-By: Tomas Krizek <tkrizek@redhat.com>