Document and check init_creds context requirement

Document and check init_creds context requirement

To ensure that the same clpreauth plugin modules and moddata pointers
are used for each step of an initial creds operation, the caller must
use the same library context for krb5_init_creds_init(),
krb5_init_creds_step(), and krb5_init_creds_free().  Document and
enforce this requirement.

ticket: 7877