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/version-script.map

@@ -83,6 +83,7 @@ HEIMDAL_KRB5_2.0 {
 		krb5_cc_cache_next;
 		krb5_cc_clear_mcred;
 		krb5_cc_close;
+		krb5_cc_configured_default_name;
 		krb5_cc_copy_cache;
 		krb5_cc_copy_match_f;
 		krb5_cc_default;