CURLOPT_HEADERFUNCTION and CURLOPT_WRITEFUNCTION more consistent

- SMTP falls back to RFC821 HELO when EHLO fails (and SSL is not required).
- Use of true local host name (i.e.: via gethostname()) when available, as default argument to SMTP HELO/EHLO.
- Test case 804 for HELO fallback.

  properly in angle brackets. Recipients provided with CURLOPT_MAIL_RCPT now
  get angle bracket wrapping automatically by libcurl unless the recipient
  starts with an angle bracket as then the app is assumed to deal with that
  properly on its own.

replaced tabs with spaces

  http://curl.haxx.se/mail/lib-2009-10/0024.html
  http://curl.haxx.se/bug/view.cgi?id=2944325

removed trailing whitespace

  (http://curl.haxx.se/bug/view.cgi?id=2951319)

simply check for CURLM_CALL_MULTI_PERFORM internally. This has the added
benefit that this goes in line with my long-term wishes to get rid of the
CURLM_CALL_MULTI_PERFORM all together from the public API.

Known bug #64 fixed by Constantine Sapuntzakis and Joshua Kwan in 7.20.0

Chris Conroy's RTSP followup fixes

is now a very long time ago

s/RTPDATA/INTERLEAVEDATA/

'note' in most description as it is mostly useless.

libcurl options for controlling what to get and how to receive posssibly
interleaved RTP data. Initial commit.

the code more curl-style

  versions --ftp-ssl and --ftp-ssl-reqd as these options are now used to
  control SSL/TLS for IMAP, POP3 and SMTP as well in addition to FTP. The old
  option names are still working but the new ones are the prefered ones
  (listed and documented).

  command is a special "hack" used by the drftpd server, but even though it is
  a custom extension I've deemed it fine to add to libcurl since this server
  seems to survive and people keep using it and want libcurl to support
  it. The new libcurl option is named CURLOPT_FTP_USE_PRET, and it is also
  usable from the curl tool with --ftp-pret. Using this option on a server
  that doesn't support this command will make libcurl fail.

  on FTP errors in the transient 5xx range. Transient FTP errors are in the
  4xx range. The code itself only tried on 5xx errors that occured _at login_.
  Now the retry code retries on all FTP transfer failures that ended with a
  4xx response.

  (http://curl.haxx.se/bug/view.cgi?id=2911279)