discover: Add struct load_url_result and load_task

Currently, load_url and load_url_async return a filename parameter and a
tempfile flag (indicating whether the file needs to be cleaned after
use).

Instead, encapsulate this data in a struct load_url_result, which the
caller (and async callbacks) can read the status, filename and clean
parameters.

For internal use in load_url and helpers, we add a struct load_task to
hold a pointer to the load_url_result and async data.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>