Introduce system name and domain options

Bring them up from the DMARC reporting section to the mailserver toplevel
so they become reusable for the upcoming TLSRPT integration.

We default to the first domain in the domains option, if not set
explicitly, so that `systemDomain` doesn't become a blocker for existing
setups. We still encourage picking out the intended one, which is likely
the one used for the MX hostname.

This also simplifies the DMARC reporting configuration, which doesn't
need to be so fine-grained.

Co-Authored-By: Emily <git@emilylange.de>

flake.nix

@@ -94,10 +94,6 @@
                   domains = [
                     "example.com"
                   ];
-                  dmarcReporting = {
-                    organizationName = "Example Corp";
-                    domain = "example.com";
-                  };
                 };
               }
             ];