simplify gpst_xml_or_error handling and config parsing

simplify gpst_xml_or_error handling and config parsing

* replace GPST-specific xmlnode_get_text() with global xmlnode_get_val(), and make it free existing values (simplifies config loading)
* don't deal with numeric error codes in gpst_xml_or_error, only parsing
* make gpst_xml_or_error take 2 calbacks: one for XML, one for challenge (whether in XML or JavaScript form)
* allow callbacks to take cb_data
* use all this to simplify form handling

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