Check that given string represents a valid DNS name according to RFC1123;
allowUppercase is self-explanatory;
allowWildcard means that the first DNS label may be an asterisk (*).
Lookup a DNS name on the specified DNS server, and return all IP addresses;
returned slice of addresses will be nil if host was not found;
if host is an IP address, it will be returned as such;
err will be set for all other error situations.