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.

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!

The previous separation between the fs and ui process wasn't that good. The idea was to have a `ui' process tightly sandboxed, but it was a lie actually. `ui' was one imsg away from making internet connections and accessing data on the disk, so it wasn't really limited in (almost) any way. Furthermore, having to serialize data to/from the fs proc started to become not really maneagable. As a first step to fix this situation, join the fs and ui process.

partial revert of commit d54dd8160b88709f3d243e1410a781e5de7fc187. We don't really need an abstraction over "printf-like" things. I can just use plain old FILEs. open_memstream (which conforms to POSIX.1) is what I should have used from the beginning in my mcache implementation.

saves a page to the disk. Also, changes the order of the downloads so the new one is always at the top.

Persist a generalized history of visited URL in history_file and try to keep it in sync during telescope usage and shutdown.