Add new cache collection APIs

Add new cache collection APIs

* krb5_cc_get_full_name retrieves the full type:name of a cache.
* krb5_cc_switch makes a cache the primary cache.
* krb5_cc_cache_match searches the collection for a client principal.
* krb5_free_string releases a string (for the krb5_cc_get_full_name
  result).

All of these are from Heimdal except for krb5_free_string (Heimdal uses
krb5_xfree).

ticket: 6954

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25155 dc483132-0cff-0310-8789-dd5450dbe970