www/url_handlers.c: Re-work IS_POST() & csrf logic in prefs_fmap()
www/url_handlers.c: Re-work IS_POST() & csrf logic in prefs_fmap()

Tidy up the logic to check for a POST request and valid csrf token or a
GET request.

If we get a POST request but no valid csrf token, we just fall through
and treat it like a GET.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@opentechlabs.co.uk>

ac000/receiptomatic