Fix GSS krb5 GSS_C_DELEG_FLAG ret_flags result

The krb5 gss_accept_sec_context could erroneously return
GSS_C_DELEG_FLAG in ret_flags if either:

* The token included the GSS_C_DELEG_FLAG but did not include at least
  28 bytes in the authenticator checksum.
* The initial token included the GSS_C_DELEG_FLAG but a DCE-style
  exchange was performed.

Fix both of these cases.

ticket: 7987 (new)
target_version: 1.13
tags: pullup