Do mech fallback for first SPNEGO context token

Do mech fallback for first SPNEGO context token

When producing the first SPNEGO security context token, if the first
mechanism's init_sec_context fails, fall back to a later mechanism.

This fixes a regression in 1.10 for SPNEGO initiators using non-krb5
credentials.  The identity selection work causes errors to be deferred
from krb5's acquire_cred in some cases, which means SPNEGO doesn't see
an error until it tries the krb5 init_sec_context.

ticket: 6936
target_version: 1.10
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25591 dc483132-0cff-0310-8789-dd5450dbe970