it's just a uri and some headers now. those can be function arguments
with no loss of clarity. *actual* additional arguments, for example a
TLS context with additional certificates, could be added on a new and
improved FileTransfer class that carries not just a backend reference
but some real, visible context for its transfers. curl not being very
multi-threading-friendly when using multi handles will make sharing a
bit hard anyway once we drop the single global download worker thread
Change-Id: Id2112c95cbd118c6d920488f38d272d7da926460
eldritch horrors
6f18e1ebde