environ in the authentication callback functions.

- auth.basic was modified to have a callback of
  authfunc(environ, username, password)

- auth.digest was modified in a similar manner,
  authfunc(environ, realm, password)

- auth.digest's digest_password also had it's
  arguments reversed to be consistent with the
  corresponding authfunc(); if you're going to
  break -- let's fix two things at once!

- auth.form has a change similar to auth.basic

These changes were suggested via Matthew Scott
on the paste mailing list; only that I put the
environ first to be consistent with other
WSGI functions.