Add ISC_{LIST,LINK}_INITIALIZER for designated initializers [v9.18]

Since we are using designated initializers, we were missing initializers for ISC_LIST and ISC_LINK, add them, so you can do

*foo = (foo_t){ .list = ISC_LIST_INITIALIZER };

Instead of:

*foo = (foo_t){ 0 };
ISC_LIST_INIT(foo->list);

Backport of !6928 (merged)