The dovecot filter config distributed with fail2ban watches for log
entries with a journalmatch filter of "_SYSTEMD_UNIT=dovecot.service",
but NixOS's Dovecot runs as dovecot2.service. Fix the journalmatch
config to match NixOS's Dovecot.

Motivation for this change

fail2ban doesn't currently work with dovecot because the upstream config uses a different name for the dovecot systemd service than NixOS.

Things done

I'm afraid I am very new to Nix / NixOS and I'm not sure how to test this.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.