Revision of telnet-brute by nnposter.

http://seclists.org/nmap-dev/2013/q2/101

- Changed line termination from \r\0 to \r\n.

- Changed response to the telnet server ECHO "will" / "will not" from
  outright "do not" to an agreement with whatever the server proposes
  to make the script work with some daemons.

- Removed use of receive_lines(), which was causing either time-outs or
  unnecessary connection tear-downs due to waiting on a line
  termination. This change improved the script success rate and/or
  performance (5x in some cases).

- Exposed the connection time-out value as a configurable parameter
  (telnet-brute.timeout). It defaults to 5s.

- Improved handling of connection errors, which were occassionally
  causing credential combinations to be skipped.

- Updated the logged-in status detection logic to make the script work
  with some daemons.

- Avoided overlapping connections to make the script work with daemons
  that allow only one connection at a time.

- Replaced a locally defined routine with stdnse.string_or_blank() for
  printing out credentials. Changed printing of tested credentials in
  the debug output to be consistent with script results.

- Script will now report if it senses password-only authentication.

- Implemented detailed debug messages (e.g. "Sending password") at
  debug level 3 (configurable).

- Expanded the script documentation.