The default prepare script now uses appropriate options if it detects Postgres
12 or higher. The error handling still needs work though.

Update prepare script for newer Postgres

Our backup logs are generally full of

+ ssh -p 22 -o ConnectTimeout=10 root@openvpn2020.ch.cam.ac.uk '
  umask 077
  FILELIST=`tempfile`
  CONFLIST=`tempfile`
  mkdir -p /var/adm/backup
  # Make logrotate use datestamps
  if ! grep -q dateext /etc/logrotate.conf ; then sed  -i '\''s/^include/dateext\ninclude/'\'' /etc/logrotate.conf ; fi
  # Which packages are installed?
  dpkg --get-selections | awk '\'' { print $1 ; } '\'' >/var/adm/backup/packages
  cat /var/lib/dpkg/info/*.list | while read F ; do [ -f "$F" ] && echo "$F" ; done | sort > $FILELIST
  awk '\''/Description:/ { flag = 0 } ; flag == 1 { print $1 ; }  ; /Conffiles:/ { flag = 1  } ; '\'' </var/lib/dpkg/status |  sort >$CONFLIST
  diff -u $FILELIST $CONFLIST | grep ^-/ | sed s/^-// >/var/adm/backup/package-files
  rm
 '
rm: missing operand

because we're not escaping the args to that final rm command! This means we gradually fill
up /tmp, especially on machines that are infrequently rebooted (e.g. calculon)