Rework gpp_cred_handle management
Provide helper functions to create and free gpp_cred_handle, and also record
if the credential is a default credential or there is a custom ccache name
associated with it.
This is in preparation of following patches.
Signed-off-by: Simo Sorce <email@example.com>
[firstname.lastname@example.org: style fixups, typos in commit message]
Reviewed-by: Robbie Harwood <email@example.com>