Mailserver v1.4.1

Patch level release; minor improvements:

1. fix a minor issue with dns_check.sh
2. Add mailserver version to the main dashboard.

NOTE

• when upgrading from 1.3.x or lower, please read https://github.com/t-lo/mailserver/releases/tag/mailserver-v1.4.0.
• when upgrading from 1.2.x or lower, please read https://github.com/t-lo/mailserver/releases/tag/mailserver-v1.3.0.