steam-api: use JSON over XML with BitlBee's internal parser
steam-http: when freeing SteamHttpReqs directly close the http_request

BitlBee 3.2 introduces a new function, http_close(), which can be used
to directly free and close an http_request on the spot. This stops from
having to set a "fake" callback so that http_request is cleaned up.
configure: bump required bitlbee version to 3.2

BitlBee 3.2 is now needed for the JSON interface and http_close().