s4-credentials Add explicit event context handling to Kerberos calls (only)

s4-credentials Add explicit event context handling to Kerberos calls (only)

By setting the event context to use for this operation (only) onto
the krb5_context just before we call that operation, we can try
and emulate the specification of an event context to the actual send_to_kdc()

This eliminates the specification of an event context to many other
cli_credentials calls, and the last use of event_context_find()

Special care is taken to restore the event context in the event of
nesting in the send_to_kdc function.

Andrew Bartlett