Thanks to Luis Carvalho for spotting the error.

Commit 51967f98 began treating SERVFAIL
as a successful response from an upstream server (thus ignoring future
responses to the query from other upstream servers), but a typo in that
commit means that REFUSED responses are accidentally being treated as
successful instead of SERVFAIL responses.

This commit corrects this typo and provides the behaviour intended by
commit 51967f98: SERVFAIL responses are considered successful (and will
be sent back to the requester), while REFUSED responses are considered
unsuccessful (and dnsmasq will wait for responses from other upstream
servers that haven't responded yet).

Also return REFUSED and not SERVFAIL when out of memory.

Thanks to Allain Legacy for problem report.