some ban actions need additional packages (eg ipset). since actions can be
provided by the user we need something general that's easy to configure.
we could also enable ipset regardless of the actual configuration of the system
if the iptables firewall is in use (like sshguard does), but that seems very
clumsy and wouldn't easily solve the binary-not-found problems other actions may
also have.
it's not possible to set a different default maxretry value in the DEFAULT jail
because the module already does so. expose the maxretry option to the
configuration to remedy this. (we can't really remove it entirely because
fail2ban defaults to 5)
I'm not really sure which one of types.lines or types.str that fit
better, but I'm going for types.lines because it behaves more like the
current type (i.e. have the ability to merge).
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
