Make gss_store_cred*() work

krb5_cc_cache_match() searches all ccache collections for a ccache that
has credentials for a given principal name.  This includes MEMORY
ccaches, which means it can find the same ccache as is referenced by a
GSS cred handle given to gss_store_cred(), which means that
gss_store_cred() can fail.

For now we work around this by including a private variant of
krb5_cc_cache_match() that only searches the default ccache, not all
collections.  Eventually we should ensure that krb5_cc_default() also
searches all collection-type (other than MEMORY) ccaches for a default
credential, then we can go back to using krb5_cc_cache_match() (though
we'll need to make sure that MEMORY is searched last or not at all).

lib/gssapi/mech/gss_store_cred.c

@@ -33,6 +33,7 @@
 
 #include "mech_locl.h"
 
+/* See RFC5588 */
 GSSAPI_LIB_FUNCTION OM_uint32 GSSAPI_LIB_CALL
 gss_store_cred(OM_uint32         *minor_status,
 	       gss_cred_id_t     input_cred_handle,