Update courier ports to latest version.

[madpilot]

Summary

Due to changes in courier-imap it is required to update these ports in one go.

I've also added an UPDATING message, copying from the ipstream updating instructions for the maual steps required from users.

Test Plan

I'm already using the authlib, imap and maildrop parts myself with succes. I'm unable to test cone and sqwebmail myself.

[madpilot]

Add back patch-configure removed by mistake.

[oliver]

Looks fine for me.