dns_ip4 looks up 4-byte IP addresses for the fully-qualified domain name in
fqdn. It puts the concatenation of the IP addresses into out and
returns 0. If the domain does not exist in DNS, or has no IP addresses,
out will be empty.
If dns_ip4 has trouble with the DNS lookup or runs out of memory, it returns
-1, setting errno appropriately. It may or may not change out.
If fqdn is a dotted-decimal IP address, dns_ip4 puts that IP address into
out without checking DNS. More generally, if fqdn is a
dot-separated sequence of dotted-decimal IP addresses, dns_ip4 puts those IP
addresses into out without checking DNS. Brackets may appear inside the
dotted-decimal IP addresses; they are ignored.