Pass a CURL * pointer in dephandle to identify the stream within the
same connection that this stream is depending upon. This option clears the
exclusive it and is mutually exclusive to the
The spec says "Including a dependency expresses a preference to allocate
resources to the identified stream rather than to the dependent stream."
This option can be set during transfer.
dephandle must not be the same as handle, that will cause this
function to return an error. It must be another easy handle, and it also needs
to be a handle of a transfer that will be sent over the same HTTP/2 connection
for this option to have an actual effect.