was broken after the fs/ui merge

the recent commit to rework handle_save_page_path changed the tab id *before* setting it to the download, so the ui and net process went out of sync and downloads hanged.

must_select otherwise is set when MB_COMPREAD but not cleared on MB_READ, yielding funny errors. While here, move also the other parts of the state out of the if (MB_COMPLREAD) to avoid this kind of issues in the future.

it's unused as write_buffer now writes a buffer to the disk.

there's no need to handle the buffer case, which is now handled directly by write_page. While here fix an issue if the open fails: we still need to stop the tab!

I can't use the ""generate temp file + rename" trick used for the session file, because I only add items that are not "dirty". the history-on-disk things needs some improvements, mostly to limitate the size of the file, but for the time being unbreak it!