Add the DIR ccache type

Add the DIR ccache type

The DIR ccache type supports a collection of credential caches within
a private directory (which must be created out of band).  One cache is
designated as primary at any given time.  Setting the default cache
name to DIR:dirname will cause caches within dirname to be present in
the global cache collection.

ticket: 6953

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25154 dc483132-0cff-0310-8789-dd5450dbe970