Fix dnstap output file rolling

See RT#46942.

This MR contains the contents of the rt46942 branch with @marka's comments from RT addressed.

@marka also wrote:

I would be tempted to make dns_dt_reopen perform the task send rather than call it from the event handler.

I think this is a good idea, but while implementing it I came across a bug in how dns_dt_reopen() handles the roll argument passed from named_server_dnstap() that I would need to fix first. As I strongly feel we should get RT#46942 fixed in 9.11.3 and 9.12.1, I refrained from implementing @marka's suggestion quoted above in this MR. I will open another MR to fix the aforementioned bug in dns_dt_reopen().