support more than one endpoint

[ennorehling]

get_prefix doesn't do anything useful that nginx cannot already do with the fastcgi_split_path_info command.

[ennorehling]

actually, other tasks are talking about adding different sub-urls (k/ for the data store, a/ for an admin interface, l/ for login, s/ to show a list of all keys, so this function may still be useful in the future.