When finishing the NSSEARCH task and there is no more followup
lookups to start, dig does not destroy the last lookup, which
causes it to hang indefinitely.
Rename the unused `first_pass` member of `dig_query_t` to `started`
and make it `true` in the first callback after `start_udp()` or
`start_tcp()` of the query to indicate that the query has been
started.
Create a new `check_if_queries_done()` function to check whether
all of the queries inside a lookup have been started and finished,
or canceled.
Use the mentioned function in the TRACE code block in `recv_done()`
to check whether the current query is the last one in the lookup and
cancel the lookup in that case to free the resources.
7d360bd05e