libhtml: fix potential linked list corruption

libhtml: fix potential linked list corruption

the difficulty is freeing items while parsing because items might already
be linked into various linked lists like in docinfo.images or form.fiels.
so we link images, tables and formfields to the docinfo as the final step
of getitems() pass using the new recursive function linkitems(). as only
reachable items get linked theres no danger of dangeling pointers.