Add krb5_cc_configured_default_name()

Refactor krb5_cc_set_default_name() by splitting out the part that looks
for a configured default ccache name.  This will allow one to check if a
given ccache is a default ccache for a process ignoring KRB5CCNAME,
which might prove useful in the kx509 client.

lib/krb5/krb5_locl.h

@@ -273,6 +273,7 @@ typedef struct krb5_context_data {
     krb5_addresses *ignore_addresses;
     char *default_cc_name;
     char *default_cc_name_env;
+    char *configured_default_cc_name;
     int default_cc_name_set;
     HEIMDAL_MUTEX mutex;		/* protects error_string */
     int large_msg_size;