Fixes#22575, as suggested by poettering in #35514.
Intended as a workaround for some buggy routers, which refuse to send empty
replies. If systemd-resolved starts two DnsTransactions, one for A and one
for AAAA, and the domain in question has no AAAA entry, then the server will
send a reply for A and no reply for AAAA. Correct behavior for the server would
be to send an empty reply for AAAA.
systemd-resolved would previously keep retrying the AAAA transaction, and
eventually timeout the whole query, returning an error to the caller.
Now, if the server replies to one query and not another, we cut short the
timeout and return the partial result. Returning the partial result allows
the rest of the system to keep working. It matches how e.g. glibc libnss_dns
behaves.
0da73fab56