bugfix for OTP "challenge" form handling

bugfix for OTP "challenge" form handling

In the patch entitled 'Recognise auth forms named "challenge" as token
requests' (commit 51f8feb6, released in v8.00) the condition for using an
OTP token in an AnyConnect login form was changed from:

  (field is named `secondary_password`)

… to:

  (field is named `secondary_password`) AND (form is named `challenge`)

This was almost certainly a mistake, and should have been as follows:

  (field is named `secondary_password`) OR (form is named `challenge`)

This patch rewrites the condition to do just that, in a clearer form, and
should fix GitLab issue #24 (https://gitlab.com/openconnect/openconnect/issues/24#note_157035052).

Signed-off-by: Daniel Lenski <dlenski@gmail.com>