New static error: options in the XProc namespace

One consequence of using variables instead of functions for the state
information ($p:episode and friends) is that they potentially conflict
with the names of options.

That is,

  <p:parameter name="foo" select="$p:episode"/>

would refer to the currently in-scope option named "p:episode" today.

I've added a new static error forbidding the creation of options in
the XProc namespace to avoid this problem.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Clearness is so eminently one of the
http://nwalsh.com/            | characteristics of truth that often it
                              | even passes for truth itself.-- Joubert