Pass a CURL * pointer in dephandle to identify the stream within the
same connection that this stream is depending upon exclusively. That means it
depends on it and sets the Exclusive bit.
The spec says "Including a dependency expresses a preference to allocate
resources to the identified stream rather than to the dependent stream."
Setting a dependency with the exclusive flag for a reprioritized stream causes
all the dependencies of the new parent stream to become dependent on the
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.