CURL_PROGRESSFUNC_CONTINUE not defined

[StefanKarpinski]

The progress callback is supposed to indicate success by returning the special values CURL_PROGRESSFUNC_CONTINUE. However, this is is not defined or exported by the LibCURL bindings.

[StefanKarpinski]

I misunderstood the docs: it turns out that returning this value causes libcurl to also call the built-in progress callback after this one, which is generally not something one wants, so this is not super useful, but should probably still be defined and exported, so I'll leave this issue here.