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_acquire_cred_from.c

@@ -179,14 +179,12 @@ gss_acquire_cred_from(OM_uint32 *minor_status,
     } else
 	mechs = _gss_mech_oids;
 
-    cred = calloc(1, sizeof(*cred));
+    cred = _gss_mg_alloc_cred();
     if (cred == NULL) {
 	*minor_status = ENOMEM;
 	return GSS_S_FAILURE;
     }
 
-    HEIM_SLIST_INIT(&cred->gc_mc);
-
     if (actual_mechs) {
 	major_status = gss_create_empty_oid_set(minor_status, actual_mechs);
 	if (GSS_ERROR(major_status))