nss-util: be a tiny bit more compatible with glibc's lookup behaviour regarding IPv6

Check for RES_USE_INET6 before we prefer IPv6 over IPv4, for all our NSS
modules. (Not that the DNS resolver that is configured with this matters
to us, but hey, let's try to be compatible).
resolved: don't trip up when an rtlink message does not include the MTU
resolved: add DNS cache
update TODO