rename callback() to cc_plugin_register_to_context() and document
its purpose.
The parameter that is the cc_ops structure is 'plugctx' not 'plug'.
This is critical because 'plug' is the common_plugin_model structure
and casting the wrong object leads to random behavior including
crashes.
The test to exit without action was reversed.
With these changes ccache plugins can be registered on each
krb5_init_context() operation.
Change-Id: I295ea91759f69b36ac13b1bfff87306d40df4a26
