Host names and underscore

[richardschneider]

A hostname cannot contain an underscore (_). ServiceProfile should replace "_" with "-" when generating a default host name.

Helps with #64

[gilzad]

I assume there is an issue with the host name in the SRV record. ServiceProfile.cs creates the same string for the SRV-hostname and for the Type-A-domain-name. Avahi and Bonjour place the machine's actual host name in the SRV record, if you don't choose to overwrite it.

For my own purposes I changed ServiceProfile() to this older version, which treats Type-A-domain-name and SRV-host-name differently:

public ServiceProfile(string instanceName, string serviceName, ushort port, IEnumerable<IPAddress> addresses = null, string hostName = null)
{
  InstanceName = instanceName;
  ServiceName = serviceName;
  var fqn = FullyQualifiedName;

  var simpleServiceName = new DomainName(ServiceName.ToString()
    .Replace("._tcp", "")
    .Replace("._udp", "")
    .TrimStart('_'));//allowing underscores again

  string domainName = DomainName.Join(InstanceName, simpleServiceName, Domain);
  //host name now defaulting to computer's host name.
  HostName = hostName ?? (Environment.GetEnvironmentVariable("COMPUTERNAME") ?? Environment.GetEnvironmentVariable("HOSTNAME"))  + "." + Domain;
  Resources.Add(new SRVRecord
  {
    Name = fqn,
    Port = port,
    Target = HostName
  });

  //removed default "txtvers=1" TXTRecord.

  foreach (var address in addresses ?? MulticastService.GetLinkLocalAddresses())
  {
    Resources.Add(AddressRecord.Create(domainName, address));
  }
}

I didn't create a PR yet for several reasons.

• Opposed to #69 we really do need the underscores in the service name for our use cases. Whilst Avahi and Bonjour also do allow underscores, I'm still not sure if this should apply for everyone, so I'm holding back here.
• I introduced a fifth parameter which might be too many. I'd rather populate a clear instance of ServiceProfile one by one, but there's no public member for Port yet. And still the SRV-hostname will be created with DomainName.Join(InstanceName, ServiceName, Domain) before I can overwrite it with a different value.
• Opposed to https://github.com/richardschneider/net-mdns/issues/13#issuecomment-403406793 I think the application layer should decide on whether to add a default "txtvers=1" or not.

However, that doesn't mean I'm not willing to contribute. I'm just careful with my current 'solution'.