barrier: fix race in test-code

The barrier_wait_next_twice* test-cases run:
  Parent:                             Child:
    set_alarm(10)                       sleep_for(1);
    ...                                 set_alarm(1);
    sleep_for(2)                        ...

Therefore, the parent exits after 2+ periods, the client's alarm fires
after 2+ periods. This race turns out to be lost by the child on other
machines, so avoid it by increasing the parent's sleep-interval to 4. This
way, the client has 2 periods to run the barrier test, which is far more
than enough.
resolve: add llmnr responder side for UDP and TCP

Name defending is still missing.
resolved: when resolving an address PTR record via llmnr, make a tcp connection by default
Update TODO
resolved: discard more invalid llmnr messages
networkd: fix reporting errors from hostnamed

The return value may be -EINVAL or a positive errno from the dbus
message. Check both ranges, otherwise most errors are silently ignored.
resolved: we don't need the DNS server "source" concept anymore, remove it
sd-network: make socket filter programs static const where possible

(also, fix some whitespace/indentation issues, and avoid "index" as identifier in
order to not clash against libc's "index()" call)
resolved: set LLMNR TCP and UDP TTLs to the values suggested by the RFC
resolved: add more debug logging
resolved: the llmnr destination address check applies to queries, not to responses
resolved: when answer A or AAAA questions, order responses by whether addresses are link-local or not
resolved: never attempt negative caching of SOA records
resolved: only cache answer RRs, never additional or authoritative RRs of responses
resolved: follow more closely the recommend timeouts and TTLs from the LLMNR spec
resolved: don't do llmnr on interfaces lacking multicasting
update TODO
resolved: don't allow adding of ANY class/type RRs to local zones
resolved: don't accept messages with ANY RRs
resolved: respond to ANY queries from our zone