Extend .debug.* flags and hide them by default

Allow to override DNS resolver address via the -debug.dnsoverride flag
and SMTP port via -debug.smtpport.

All flags are not available unless maddy is built using the 'debugflags'
tag.

internal/target/remote/debugflags.go

@@ -0,0 +1,9 @@
+//+build debugflags
+
+package remote
+
+import "flag"
+
+func init() {
+	flag.StringVar(&smtpPort, "debug.smtpport", "25", "SMTP port to use for connections in tests")
+}