Fix gssrpc data leakage [CVE-2014-9423]

Fix gssrpc data leakage [CVE-2014-9423]

[MITKRB5-SA-2015-001] In svcauth_gss_accept_sec_context(), do not copy
bytes from the union context into the handle field we send to the
client.  We do not use this handle field, so just supply a fixed
string of "xxxx".

In gss_union_ctx_id_struct, remove the unused "interposer" field which
was causing part of the union context to remain uninitialized.

ticket: 8058 (new)
target_version: 1.13.1
tags: pullup