Install etc/upgrade/ for the benefit of rt-setup-database

Without etc/upgrade/ only some actions that sbin/rt-setup-database
provides are available from an install; the rest need to be run from an
RT tarball or git checkout.  It's more annoying to setup a new RT
instance for an upgrade, test it, and then do the cutover when you can't
run `sbin/rt-setup-database --action upgrade` repeatedly from your
known-good config and install.

We install etc/acl.* and etc/schema.* already, so there's precedent for
installing schema and ACL data files.
Merge branch '4.0/install-etc-upgrade' into 4.0-trunk