Move SMTP auth setup to ConfigLoad()

This has several benefits:
- Configuration errors are caught at startup rather than upon a connection
- mailHandler() has less work to do for each connection

Rather than relying on remote_user and remote_pass to control whether
authentication is used, introduce an explicit "none" type for
remote_auth, and make that the default. (This is effectively the same
default behavior since remote_user and remote_pass default to empty.)

Also, we are in a better position to more thoroughly check for
configuration errors or mismatches:
- If remote_auth is given, remote_user and remote_pass are required.
- If remote_auth is given, remote_host is also required (because it
  makes no sense to say we're going to authenticate if we have no server
  to which to authenticate.)
- If remote_user or remote_pass are given, remote_auth cannot be "none".

main.go

@@ -4,7 +4,6 @@ import (
 	"crypto/tls"
 	"fmt"
 	"net"
-	"net/smtp"
 	"net/textproto"
 	"os"
 	"strings"
@@ -170,20 +169,6 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 
 	logger.Info("delivering mail from peer using smarthost")
 
-	var auth smtp.Auth
-	host, _, _ := net.SplitHostPort(*remoteHost)
-
-	if *remoteUser != "" && *remotePass != "" {
-		switch *remoteAuth {
-		case "plain":
-			auth = smtp.PlainAuth("", *remoteUser, *remotePass, host)
-		case "login":
-			auth = LoginAuth(*remoteUser, *remotePass)
-		default:
-			return smtpd.Error{Code: 530, Message: "Authentication method not supported"}
-		}
-	}
-
 	env.AddReceivedLine(peer)
 
 	var sender string
@@ -196,7 +181,7 @@ func mailHandler(peer smtpd.Peer, env smtpd.Envelope) error {
 
 	err := SendMail(
 		*remoteHost,
-		auth,
+		remoteAuth,
 		sender,
 		env.Recipients,
 		env.Data,