dh_installinit: Add --restart-after-upgrade, which avoids stopping a daemon in the...

dh_installinit: Add --restart-after-upgrade, which avoids stopping a daemon in the prerm, and instead restarts it in the postinst, keeping its downtime minimal. Since some daemons could break if files are upgraded while they're running, it's not the default. It might become the default in a future (v8) compatability level. Closes: #471060