Add rate limit detection to starttls-smtp

[fluggelgleckheimlen]

Some smtp hosts have a strict connection limit (10 per second or less) and sslscan --starttls-smtp returns multiple socket errors:

ERROR: The host mx1.cesactivationprod.iphmx.com on port 25 did not appear to be an SMTP service.
ERROR: Could not connect.

Instead of the original smtp message:

421 #4.4.5 Too many connections from your host.

This can be reproduced using the Cisco CES servers:

sslscan --starttls-smtp mx1.cesactivationprod.iphmx.com

It would be great if you add rate limit warning and stop the test when the 421 code appears.