Install drop-ins in /lib/ instead of /etc/, use aliases instead of overrides for alternative names

Packages should prefereably install configuration in /lib/, so that users can install local overrides in /etc/ that take precedence, and so that drop-ins are removed on package removal instead of being left around for non-existing units.

systemd natively supports alternative names for units, via the Alias= option. This package is the only one in the distribution that makes use of alternatives for this purpose, which does not fit with the standard behaviour and expectations regarding units. Alternatives are not natively supported and understood by systemd, and are not integrated (eg: no reload is performed when they switch, relationships are not clearly displayed, and actual aliasing is not performed, instead they are full copies that run the exact same commands again), using the native mechanism is preferred and recommended by the Debian systemd maintainers.

iptables-persistent.service, ip6tables-persistent.service and ipset-persistent.service are now aliases instead of alternatives, using native functionality to provide alternative names. Users wishing to use such names can use 'systemctl enable netfilter-persistent.service' to enable them, and can override them using the standard systemd configuration mechanisms. Other packages wishing to provide the same service names simply have to declare the same aliases in their units, and users can enable the one they prefer.