Use XDG-compliant paths unless ~/.telescope already exists. Provide a script to migrate to XDG-style directory for users who wish to do so.

it's been a while since we're using a lockfile to protect against multiple telescope instances.

During safe_mode the fs process shouldn't open or unlink the crashed_file.

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.