This method parses the Host field in the service. It returns 1 or 3
things 1) it is a host 2) an IPv4 address or an 3) IPv6 address.
This simplifies some code a bit and allows for 1 way of parsing the Host
field.

This *only* parse the Host field, Mail and/or Text values should be
checked separately.

We reuse the dns.TypeXXX values for this as to not invent anything new.