Fix import_sec_context with interposers

The code was correctly selecting the mechanism to execute, but it was
improperly setting the mechanism type of the internal context when the
selected mechanism was that of an interposer and vice versa.

When an interposer is involved the internal context is that of the
interposer, so the mechanism type of the context needs to be the
interposer oid.  Conversely, when an interposer re-enters gssapi and
presents a token with a special oid, the mechanism called is the real
mechanism, and the context returned is a real mechanism context.  In
this case the mechanism type of the context needs to be that of the
real mechanism.

ticket: 7592
target_version: 1.11.2
tags: pullup