s4:credentials Add hooks to extract a named Kerberos credentials cache

s4:credentials Add hooks to extract a named Kerberos credentials cache

This allows the integration of external tools that can't be linked
into C or python, but need to authenticate as the local machine
account.

The machineaccountccache script demonstrates this, and debugging has
been improved in cli_credentials_set_secrets() by passing back and
error string.

Andrew Bartlett