Fix interposed gss_accept_sec_context()

If gss_accept_sec_context() is interposed, selected_mech will be an
interposer OID.  In this situation, pass the corresponding public OID
to gss_inquire_attrs_for_mech() to determine whether the mech is
allowed by default.

[ghudson@mit.edu: pared down from larger commit; rewrote commit message]

https://github.com/krb5/krb5/commit/0b43d10333f4c4b29896cebc9447d8866b661217
Author: Robbie Harwood <rharwood@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 0b43d10333f4c4b29896cebc9447d8866b661217
Branch: master
 src/lib/gssapi/mechglue/g_accept_sec_context.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)