Better handle failures to resolve client keytab

Better handle failures to resolve client keytab

In krb5_gss_acquire_cred(), treat failure to resolve the client keytab
similarly to a client keytab which resolves but does not exist or has
no entries.  The client keytab could fail to resolve if its name
contains %{username} and the current process is acting on behalf of
the NSS system.

[ghudson@mit.edu: rewrote commit message; changed tracing call to use
a macro; cleared error message when ignoring krb5_kt_client_default()
error; added test case]

ticket: 8462 (new)