https://tickets.ch.cam.ac.uk/rt/Ticket/Display.html?id=211460

e.g. RT 211460, 211465. spri-musuem-rt-2025 had been set up with an adhoc script
which was not properly tidying up after itself due to bailing early on a "set -e"
error.

I don't think there's a sensible default quota; the value for
a workstation will be very different to a tiny VM, for example.

This is needed on focal if a client is to be able to access snapshots over NFS.
From the docs I don't see why we didn't also need this option on xenial,
but empirically, we need it on focal. (e.g. RT-207229)

We now e.g. have a cron job on cerebro-backup which calls
these scripts, where /sbin is not on the $PATH.

The new-backup-rsnapshot script understands a 'postgres' argument, but
this set up a postgres backup in an old style that we no longer use.
This change updates it to do some of the work of setting up a new style
postgres backup and tell the user what else they might need to edit to
make it go; it varies quite a lot depending on server.

Forcing arcfour causes us not to be able to back up jessie machines.