Default client keytab name

[Sam Hartman]

Here are my thoughts:

1) I'd prefer username based to uid-based for the default. That's easier
to deal with most of the time

2) The file extension sshould be keytab not client-keytab.
You can have a multi-part name if you like krb5.client.keytab or
whatever, but the last part should be keytab.

I agree that root would be desirable to treat specially, but don't think
that's an absolute requirement.