smtp-check: Add flag to specify local name

Some MTAs reject client connections unless the local name (used in the
HELO/EHLO command) looks like an FQDN. Currently, smtp-check always uses
`localhost`, which does not look like an FQDN.

This patch adds a command line flag to smtp-check to specify the
local name to be used.

Fixes https://github.com/albertito/chasquid/issues/37.

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Minor edits to the commit message, adjust flag name, go fmt.

cmd/smtp-check/smtp-check.go

@@ -25,6 +25,8 @@ import (
 var (
 	port = flag.String("port", "smtp",
 		"port to use for connecting to the MX servers")
+	localName = flag.String("localname", "localhost",
+		"specify the local name for the EHLO command")
 	skipTLSCheck = flag.Bool("skip_tls_check", false,
 		"skip TLS check (useful if connections are blocked)")
 )
@@ -91,6 +93,10 @@ func main() {
 			if err != nil {
 				log.Fatal(err)
 			}
+			err = c.Hello(*localName)
+			if err != nil {
+				log.Fatal(err)
+			}
 
 			config := &tls.Config{
 				// Expect the server to have a certificate valid for the MX