Preserve the request buffer across async SIG(0) processing

Preserve the request buffer across async SIG(0) processing

For SIG(0)-signed requests, view matching is offloaded and the request
is finished asynchronously from ns_client_request_continue(), which
passes client->inner.buffer to dns_dt_send().  That buffer aliases the
network manager's receive buffer, only valid during the read callback,
so it may already be freed and reused, producing garbage dnstap frames
(e.g. the "upforwd" sig0-over-DoT test fails with UQ=0).

When the request is offloaded (ns_client_setup_view() returns
DNS_R_WAIT) and dnstap is enabled, copy the request buffer and point
client->inner.buffer at the copy so it survives the asynchronous hop;
free it in ns__client_reset_cb().  When dnstap is disabled there is no
async consumer of the buffer, so detach it from the receive buffer
instead.

Assisted-by: Claude:claude-opus-4-8