security.c: Curl_sec_set_protection_level tweaking

- Removed sec_prot_internal as it is now inlined in the function (this removed
  a redundant check).
- Changed the prototype to return an error code.
- Updated the method to use the new ftp_send_command function.
- Added a level_to_char helper method to avoid relying on the compiler's
  bound checks. This default to the maximum security we have in case of a
  wrong input.