libkadm5clnt: leak and/or use-after-free of ccache #1331

Describe the bug
The API contract for kadm5_init_with_creds and kadm5_init_with_creds_ctx is unclear on whether the kadm5 handle or the caller owns the given ccache.

• If the kadm5 handle owns the ccache, then krb5_cc_close is missing from kadm5_c_destroy and it will leak:
  

  heimdal/lib/kadm5/destroy_c.c

  Lines 43 to 56 in abd35b2

  	free(context->realm);
  	free(context->readonly_admin_server);
  	free(context->admin_server);
  	if (context->sock != rk_INVALID_SOCKET)
  	rk_closesocket(context->sock);
  	if (context->client_name)
  	free(context->client_name);
  	if (context->service_name)
  	free(context->service_name);
  	if (context->ac != NULL)
  	krb5_auth_con_free(context->context, context->ac);
  	if(context->my_context)
  	krb5_free_context(context->context);
  	free(context);

• If the caller owns the ccache, then _kadm5_connect -> kadm_connect may free it, leading the caller to use-after-free:
  

  heimdal/lib/kadm5/init_c.c

  Line 665 in abd35b2

  krb5_cc_close(context, cc);

To Reproduce

1. Adapt kadmin(8) to krb5_cc_resolve and krb5_cc_close in an attempt to address kadmin(8) ignores existing ccache #1328.
2. Observe SIGSEGV in krb5_cc_close because libkadm5clnt already did it.

Expected behavior
Either the kadm5 handle owns the ccache, or the caller owns the ccache, and the system is clear on which is which.

Screenshots
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Desktop (please complete the following information):
NetBSD 9

Smartphone (please complete the following information):
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