---+ Client authentication to access restricted services

In the default setup, some services have restricted access.  Restriction
is provided by authorized/1 from user(user_db).  There are three ways to
autheticate a user:

  1. By logging in as a local user.  This causes ClioPatria to associate
  the current session with the local user.
  2. By logging in through OpenID.  This too causes ClioPatria to associate
  the OpenID with the current session.
  3. Using HTTP =basic= (clear password) authentication for registered
  local users.

Exploiting (1) above, clients may use /servlets/login and cookie
management to access restricted services. Alternatively, they may
exploit (3) and send basic authetication. Note however that the system
does not _challenge_ the client, and the client should thus send the
authentication data voluntarily. When using the *wget* program we can
authenticate using this command:

  ==
  wget -O - --auth-no-challenge --http-user=User --http-passwd=Passwd ...
  ==

The example below uploads a local file _file_ using *curl* to a
cliopatria server running on host cliopatria.com. The data is stored in
the _|named graph|_ =|http://my.org/rdf/|=

  ==
  curl -u User:Password -F data=@file -F baseURI="http://my.org/rdf/" \
	http://cliopatria.com:3020/servlets/uploadData
  ==

@bug The current data API is, as far as possible, compatible to Sesame.
     Unfortunately, this doesn't match ClioPatria very well. There is
     only one repository, but there are multiple named graphs.