gssapi: import mechglue allocation utility functions from Heimdal-520

Apple's Heimdal impelmentation uses a number of utility functions for
allocating names and credentials, to avoid calling malloc or calloc directly.
Import them.

lib/gssapi/mech/gss_cred.c

@@ -133,13 +133,12 @@ gss_import_cred(OM_uint32 * minor_status,
 	return GSS_S_FAILURE;
     }
 
-    cred = calloc(1, sizeof(struct _gss_cred));
+    cred = _gss_mg_alloc_cred();
     if (cred == NULL) {
 	krb5_storage_free(sp);
 	*minor_status = ENOMEM;
 	return GSS_S_FAILURE;
     }
-    HEIM_SLIST_INIT(&cred->gc_mc);
 
     *cred_handle = (gss_cred_id_t)cred;