Add ISC_{LIST,LINK}_INITIALIZER for designated initializers

Add ISC_{LIST,LINK}_INITIALIZER for designated initializers

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);

(cherry picked from commit cb3c36b8bf926e208b9817926d40cf7956f34b67)