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)

The generation of the command to unexport NFS filesystems could generate
an invalid command. Leading spaces were not being stripped, and in cases
where there is more than one backup target for a machine we need to
unexport every target. Because we also had 'set -e' in operation at this
point, the script would fail there and never clean up the moved ZFS. I
don't mind if we fail to unexport; if that's subsequently a problem for
removing the ZFS then the script will fail at that point.

This change makes the script generate better exportfs -u commands and
not exit if they fail.

The code used to open a database connection for each thread and leave
them open for as long as the scheduler ran. This worked reasonably well
until we moved to PostgreSQL 13 on Focal, although the scheduler would
fail if the database was restarted because there was no logic to
reconnect after a connection dropped.

On Focal/PG13 the connection for the 'cron' thread steadily consumes
memory until it has exhausted everything in the machine. This appears to
be a Postgres change rather than a Perl DBI change: the problem can be
reproduced by sitting in psql and running 'select * from backup_queue'
repeatedly. Once or twice a minute an instance of this query will cause
the connection to consume another MB of RAM which is not released until
the database connection is closed. The cron thread runs that query every
two seconds. My guess is it's something peculiar about the view that
query selects from - the time interval thing is interesting.
This needs more investigation.

But in the meantime I'd like to have backup servers that don't endlessly
gobble RAM, so this change makes the threads connect to the database
only when they need to, and closes the connection afterwards. This
should also make things work better over database restarts but that's
not been carefully tested.

Can't say "keys $foo", must say "keys %{$foo}" to get the keys of a hash
pointed to by a hash reference $foo. This broke with the perl in focal.

NB this is so we can put things in LOGFILE at earlier points
than the script does at present

We capture the error code at a few points in the script, and then (intend to)
log something about what went wrong. But if we set -e we immediately bail
before logging anything useful!

Resolves #1

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

For cerebro-backup where we do backups by the directory rather than the
whole server. We need to have one machine being backed up
(cerebro-filestore) but a task per directory.

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.

From the comment in the script, I think this was an attempt to ignore an
error about setting GIDs. The only place I saw that particular error
occuring in practice was for loopback-mounted isos on OS builders.
We shouldn't be backing those up in the first place! Exit code
23 is "partial transfer due to error" and so can be due to problems
other than failing to set GIDs

Split logs for tasks with same target machine but different target filesystems into
different logfiles for easier debugging.

prepare-nondebian script creates a fake excludes file for the machine now if
one doesn't exist, because I keep forgetting to do it myself for Redhat
machines and then the zfs-rsync script fails.

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