Fix two unrelated problems in SPNEGO which don't crop up with the krb5

Fix two unrelated problems in SPNEGO which don't crop up with the krb5
mechanism.

1. The third call to spnego_init_accept_context uses faulty logic to
determine if the exchange is complete, preventing a third mech token
from being sent to the acceptor if no MIC exchange is required.
Follow the logic used in the second call (in init_ctx_nego), which is
correct.

2. If the acceptor selects a mech other than the optimistic mech, it
sets sc->mic_reqd to 1 whether or not the selected mech supports MICs
(which isn't known until the mech completes).  Most code outside of
handle_mic checks sc->mic_reqd along with (sc->ctx_flags &
GSS_C_INTEG_FLAG), but the code in acc_ctx_call_acc neglected to do
so, so it could improperly delegate responsibility for deciding when
the negotiation was finished to handle_mic--which never gets called if
(sc->ctx_flags & GSS_C_INTEG_FLAG) is false.  Fix acc_ctx_call_acc to
check sc->ctx_flags so that mechs which don't support integrity
protection can complete if they are selected non-optimistically.

ticket: 6603
target_version: 1.8
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23742 dc483132-0cff-0310-8789-dd5450dbe970