security.c: sec_send tweaks

- Renamed it to do_sec_send as it is the function doing the actual
  transfer.
- Do not return any values as no one was checking it and it never
  reported a failure (added a FIXME about checking for errors).
- Renamed the variables to make their use more specific.
- Removed some casts (int -> curl_socket_t, ...)
- Avoid doing the htnl <-> nthl twice by caching the 2 results.