Add support for client keytab from cred store

The new credential store extensions added support for specifying a
specific ccache name and also a specific keytab to be used for accepting
security contexts, but did not add a way to specify a client keytab
to be used in conjunction with the Keytab initiation support added also
in 1.11

This patch introduces a new URN named client_keytab through which a
specific client_keytab can be set when calling gss_acquire_cred_from()
and Keytab Initiation will use that keytab to initialize credentials.

[ghudson@mit.edu: minor C style fix]

ticket: 7598 (new)
Add gss_import_cred_from client keytab test

Modify t_credstore.c to be more flexible and adjust t_gssapi.py
accordingly.  Add a test to t_client_keytab.py which acquire creds
using a programmatically specified client keytab.

ticket: 7598
Add krb5_kt_dup API and use it in two places

Add an API to duplicate keytab handles, mirroring krb5_cc_dup.  Use it
to simplify the krb5 GSS acquire_cred code.

ticket: 7599 (new)