Implement built-in sender restriction

[albertito]

Issue forked from #62 (Document lack of sender restriction).

Today, chasquid allows authenticated users to send emails as any other user or domain. For example, you can authenticate as a@a, and send email as b@b. This is a design choice made to balance simplicity of operation and use.

Users can implement stricter restrictions via the post-DATA hook, but that is not very easy nor intuitive.

This behaviour is now documented, as per issue #62.

It would be nice to provide a built-in mechanism to enforce sender restriction, at least for the most common use cases.

@mvolfik in #62 kindly posted some patches for doing this per-user:

btw, here's a patch that I deployed to my own server that implements this check in chasquid itself. It's patched in the obvious place with least possible effort while staying compatible with old users config file, not sure if there's any interest in merging this, but I just wanted to offer it: main...mvolfik:chasquid:main

(there's also a patch of the sender address of delivery status notifications - I'm forwarding my received emails to gmail, and it refused to accept the <> from address)