add "mox localserve" subcommand, for running mox locally for email-related testing/developing

localserve creates a config for listening on localhost for
imap/smtp/submission/http, on port numbers 1000 + the common service port
numbers. all incoming email is accepted (if checks pass), and a few pattern in
localparts are recognized and result in delivery errors.

main.go

@@ -90,6 +90,7 @@ var commands = []struct {
 	{"import mbox", cmdImportMbox},
 	{"export maildir", cmdExportMaildir},
 	{"export mbox", cmdExportMbox},
+	{"localserve", cmdLocalserve},
 	{"help", cmdHelp},
 
 	{"config test", cmdConfigTest},