sd-network: fix parameter order for sd_network_monitor_new()

Constructors should return the object they created as first parameter,
except when they are generated as a child/member object of some other
object in which case that should be first.
dns-domain: enforce maximum DNS domain name length
resolved: properly handle MTU logic
resolved: we are never authoritative for localhost
resolved: fix check for mdns names
sd-network: remove redundant array size parameter from functions that return arrays

As long as the number of array entries is relatively small it's nicer to
simply return the number of entries directly, instead of using a size_t*
return parameter for it.
sd-network: if a boolean is mising, we should just take it as false

That way, we can deprecate fields later on without problems
sd-network: rename "index" parameter to "ifindex"

makes things a bit clearer and avoids any clashes with libc's index()
symbol.
sd-login: always use "indices" as plural of "index"

So far both "indexes" and "indices" was used. Let's clean this up, and
stick to indices, since it appears to be used more frequently.