Make gss-krb5 use cache collection

Make gss-krb5 use cache collection

For default credentials, defer ccache resolution until we need the
information.  If this happens in init_sec_context when we have the
target principal in hand, use krb5_cc_select() to pick a cache.  If
the target principal is not known, use the default cache.

For credentials with a specified principal, use krb5_cc_cache_match()
to find the cache.  If no cache is found and a password is specified,
create a new cache within the collection to hold the new credentials,
if the default cache type supports a collection.

ticket: 6958

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25160 dc483132-0cff-0310-8789-dd5450dbe970