auth: Implement special handling of <select> dropdowns on XML POST

auth: Implement special handling of <select> dropdowns on XML POST

Experimentation with the Cisco AnyConnect client showed that the
following changes need to be made for compatibility:

1) If the "value" attribute is missing from the <option> node, use the
XML node content instead.  i.e. this should post as
"<dropdown>vpn</dropdown>":

    <select name="dropdown">
      <option>vpn</option>
    </select>

And this should post as "<dropdown>optname</dropdown>":

    <select name="dropdown">
      <option value="optname">vpn</option>
    </select>

2) If the name of the <select> node happens to be "group_list", put the
response in a special <group-select> node right under the <config-auth>
node, instead of putting it under the <auth> node.  (These strings are
hardcoded into the Cisco client.)

Reported-by: Fabian Jäger <fabian.jaeger@chungwasoft.com>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>