Create a file ~/.telescope/crashed on startup and unlink it on shutdown. If it's present on startup, it means that last time we crashed. It's like the ``dirty'' bit on filesystems to guard for unproper unmounts.

IMSG_GET_FILE handles a path to a local file. fs now behaves like net, it sends a IMSG_GOT_CODE/META and optionally the file. This also includes a bit of refactoring: the send_file function was extracted from handle_get so it can be used in handle_get_file too.