CONNECT-309 Just forward the relevant headers from the OnApp ControlPanel REST response,
plus CONNECT-310 use appropriate new backend interface.

oaprocci/oacontract.c
Altered: "handle_onapp_action_context()" CONNECT-310 Use "onapp_backend" interface for "update()";
CONNECT-309 "handle_alert()" to create new REST response populated with just the useful header information from the original response.

oaprocci/onapp_helpers.c
oaprocci/onapp_helpers.h
Added: "get_http_length_string()"; made dump function parameters const.