dns-domain: add calls to join/split SRV/DNS-SD service domains

dns-domain: add calls to join/split SRV/DNS-SD service domains

This adds dns_service_join() and dns_service_split() which may be used
to concatenate a DNS-SD service name, am SRV service type string, and a
domain name into a full resolvable DNS domain name string. If the
service name is specified as NULL, only the type and domain are
appended, to implement classic, non-DNS-SD SRV lookups.

The reverse is dns_service_split() which takes the full name, and split
it into the three components again.