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
resolved: remove unused variable
test-pty: silence a warning
resolved: silence warnings

No need to write to r here since it will be overwritten as the first
step in parse_fail.
sd-network: expose 'unmanaged' as a regular state

This is useful to save in the consumer of the lib, unlike ENODATA/EBUSY which
means that the user should wait until a useful state is available.
networkd-wait-online: track links

Rather than refetching the link information on ever event, we liston to
rtnl to track them. Much code stolen from resolved.

This will allow us to simplify the sd-network api and don't expose
information available over rtnl.
sd-network: fixup api

Do not expose link_is_loopback, people should just get this from rtnl directly.
Do not expose NTP servers as IP addresses, these must be strings.
Expose ifindex as int, not unsigned. This is what the kernel (mostly) and glibc uses.
test-tables: fix build-scan
sd-rtnl: make string returned by sd_rtnl_message_read_string() const
shared: rename PROTO_ADDRESS_SIZE() to FAMILY_ADDRESS_SIZE()

We mostly use "family" to refer to AF_INET, AF_INET6, etc, let's use
this terminology here, too
resolved: add LLMNR support for looking up names
Merge remote-tracking branch 'upstream/master'