MINOR: mt_lists: Give MT_LIST_ADD, MT_LIST_ADDQ and MT_LIST_DEL a return value.

MINOR: mt_lists: Give MT_LIST_ADD, MT_LIST_ADDQ and MT_LIST_DEL a return value.

Make it so MT_LIST_ADD and MT_LIST_ADDQ return 1 if it managed to add the
item, 0 (because it was already in a list) otherwise.
Make it so MT_LIST_DEL returns 1 if it managed to remove the item from a
list, or 0 otherwise (because it was in no list).