Restore the use of nftables and, for some jails, the systemd backend

Vincent Lefevrerequested to merge
vinc17/fail2ban:master into master

This reverts and fixes c03b1a83.

  • Restore banaction = nftables in the defaults-debian.conf default.
  • Instead of "backend = systemd" (which was added by e634fa86 but got removed by the aforementioned commit because it was affecting jails that do not log via syslog, thus silently finding nothing), add the systemd backend only for a known set of jails that log via syslog: list copied from paths-fedora.conf to paths-debian.conf (paths-arch.conf uses the same list). This is necessary for sshd, which is enabled in the defaults-debian.conf default, but also for other jails if they have been enabled by the user and the backend hasn't been explicitly set. Note: mysql was initially in the list, but got removed because "mysqld does not log login attempts to the journal" (see f3f813a9).

Closes: #1070677

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