acquire_cred(), kg_caller_provided_ccache_name(): On further reflection

acquire_cred(), kg_caller_provided_ccache_name(): On further reflection
and testing the correct thing appears to be to have gss_krb5_ccache_name()
stop gss_acquire_cred() from searching for the desired name in the cache
collection.  If the caller sets the ccache name then gss_acquire_cred will only
look in that ccache.  Added kg_caller_provided_ccache_name() to tell whether
or not the caller has actually set the ccache.  This should fix the problem for
both Mac OS X and Windows.

ticket: 4024

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18343 dc483132-0cff-0310-8789-dd5450dbe970