lighttpd: generate a default config file

Instead of forcing users to configure lighttpd manually, make it an
option. The current services.lighttpd.configText option can still be
used for manual configuration, but if it is left blank (default) we'll
use the new generated config file.

The generated config file ensures that the server drops root priveleges
and runs as the "lighttpd" user. It pulls in some new config params that
can be set in configuration.nix (here with default values):

  services.lighttpd.document-root = "/srv/www"
  services.lighttpd.port = 80
  services.lighttpd.extraConfig = ""  # appended to the generated file

And it enables access and error logging to the systemd journal.

Patch contributed by Bjørn Forsman.