Refresh DKIM unit
Summary:
The OpenDKIM unit has been refactored to ensure ease of installation
to FreeBSD server instead of lxc container:
- create an opendkim user, something the package doesn't do on FreeBSD
- enable the service
- avoid symbolic link for OpenDKIM extra utilities
Include DKIM unit in mailserver role.
Ref T2066.
Test Plan: Deployed to Hervil
Reviewers: DorianWinty
Reviewed By: DorianWinty
Maniphest Tasks: T2066
Differential Revision: https://devcentral.nasqueron.org/D3550