In the NSSEARCH followup lookup, when one of the queries fails to be
set up (UDP) or connected (TCP), DiG doesn't start the next query.
This is a mistake, because in NSSEARCH mode the queries are independent
and DiG shouldn't stop the lookup process just because setting up (or
connecting to) one of the name servers returns an error code in the
`udp_ready()` or `tcp_connected()` callbacks.
Write a new `nssearch_next()` function which takes care of starting the
next query in NSSEARCH mode, so it can be used in several places without
code repetition.
Make sure that the `udp_ready()` and `tcp_connected()` functions call
`nssearch_next()` in case they won't be calling `send_udp()` and
`send_tcp()` respectively, because in that case the `send_done()`
callback, which usually does the job, won't be called.
Refactor `send_done()` to use the newly written `nssearch_next()`
function.
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