Provide a few Email features in Cacti

[TheWitness]

Feature Request

Is your feature request related to a problem? Please describe

1. Limit domainless Emails to the sendmail and PHP mailer functions.
2. Provide an Email address form method to allow single emails or comma delimited emails in form input.

This will allow us to have more front end form validation instead of relying on the backend for such verification and support legacy sites using a simple username as the correct Email address which use postfix for example to append the Email address or that leverage the .forward file in the users home directory for the full Email address.

Describe alternatives you've considered

This used to work, and some years ago we broke it. This change will maintain the functionality we returned to Cacti in 1.2.31 while at the same time increasing the utility of our form API for future plugins and UI components.

Additional context

We agreed to this in our monthly developers meeting.