acct_gather_profile/influxdb - use curl_easy_getinfo to get response code.

Using curl_easy_getinfo with CURLINFO_RESPONSE_CODE is a cleaner and safer
way to retrieve the response code, since we don't have to manually parse
the header with strtok. This permits to disable the CURLOPT_HEADER option,
and thus chunk.message will directly contain the JSON response body without
the header, which can also be directly logged if more verbosity is needed to
know why a non-2xx response code was received (database not found, ...).

Bug 2693.