WIP: nixos/lighttpd: add services.lighttpd.enableModules option

This option allows looser coupling between lighttpd and its
sub-services.

While the option is a list, its purpose is to provide a the set of
needed modules to load for lighttpd to function correctly with its
config. The nixos lighttpd module ensures lighttpd modules are loaded in
the correct order and removes duplicates from the .enableModules list.

Also make the full list of modules available for users to install
(unlike now, where it's a very limited set).