Fix interposed gss_accept_sec_context()

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]

(cherry picked from commit 0b43d10333f4c4b29896cebc9447d8866b661217)

ticket: 8338
version_fixed: 1.14.1