Create a new role::mail hierarchy

Move exim- and mail-specific manifests from site.pp to a new role::mail
hierarchy.

This mostly is about sodium. Since the mchenry bits have been
traditionally unpuppetized, and since it's a bad idea to start
puppetizing such an old box right now, move the puppetized bits as an
"oldmx" temporary role.

Change-Id: I994c30ce5dea9b4ce7382ef4cfdc7482ca431f6b
mail: move clamav include in the role class

Move the clamav include in the role class, from the mail manifest, to
make it consistent with how SpamAssassin is configured. No functional
change.

Change-Id: Ibb9c1411a8fc185ceb5fadbb9a41400247f0bb6a