Make SAVECONF keep only one backup and add sandbox rules for it. #40317

When seccomp sandbox is active, SAVECONF failed because it was not
able to save the backup files for torrc. This commit simplifies
the implementation of SAVECONF and sandbox by making it keep only
one backup of the configuration file.
9 jobs for backport-mr-350 in 27 minutes and 53 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #21093
debian-disable-dirauth

00:05:28

passed #21094
debian-disable-relay

00:05:58

passed #21089
debian-distcheck

00:08:23

passed #21090
debian-docs

00:03:03

passed #21088
debian-hardened

00:08:24

passed #21091
debian-integration

00:13:39

passed #21087
debian-minimal

00:06:43

passed #21095
debian-nss

00:05:10

passed #21092
debian-tracing

00:06:43