Properly reflect MS krb5 mech in SPNEGO acceptor

r25590 changed negotiate_mech() to return an alias into the acceptor's
mech set, with the unfortunate side effect of transforming the
erroneous Microsoft krb5 mech OID into the correct krb5 mech OID,
meaning that we answer with a different OID than the requested one.
Return an alias into the initiator's mech set instead, and store that
in mech_set field the SPNEGO context.  The acceptor code only uses
mech_set to hold the allocated storage pointed into by internal_mech,
so this change is safe.

ticket: 7858
target_version: 1.12.2
tags: pullup
Test SPNEGO acceptor response to MS krb5 mech OID

In t_spnego.c, add code to make a SPNEGO request with the erroneous
Microsoft OID value and examine the response to make sure that it uses
the same OID value as the request did.  The token and tmp variables
were unused, so rename them to itok and atok for the purpose of the
new test code.

ticket: 7858
target_version: 1.12.2
tags: pullup