lighttpd: add cgit sub-service

(cgit is "a hyperfast web frontend for git repositories written in C")

cgit is enabled like this (assuming lighttpd is already enabled):

```
  services.lighttpd.cgit.enable = true;
```

and configured verbatim like this (contents of the cgitrc file):

```
  services.lighttpd.cgit.configText = ''
    cache-size=1000
    scan-path=/srv/git
  '';
```

cgit will be available from this URL: http://yourserver/cgit

In lighttpd, I've ensured that the cache dir for cgit is created if cgit
is enabled.