mda-lmtp: Handle "<>" addresses

Either the recipient or from addresses can be "<>" to indicate the null
address. mda-lmtp does not handle that well, as it sends "<<>>" which is
invalid.

This patch fixes that by special-casing them, which is unfortunate but
reasonably common.

cmd/mda-lmtp/mda-lmtp.go

@@ -84,6 +84,13 @@ func main() {
 		tempExit("Could not get hostname: %v", err)
 	}
 
+	if *fromwhom == "<>" {
+		*fromwhom = ""
+	}
+	if *recipient == "<>" {
+		*recipient = ""
+	}
+
 	cmd(tc, 250, "LHLO %s", hostname)
 	cmd(tc, 250, "MAIL FROM:<%s>", *fromwhom)
 	cmd(tc, 250, "RCPT TO:<%s>", *recipient)