disable GnuTLS insecure ciphers

Make GnuTLS old and new consistent, specify the desired protocol, cipher
and certificate type in always in both modes. Disable insecure ciphers
as reported by howsmyssl.com. Honor not only --sslv3, but also the
--tlsv1[.N] switches.

Related Bug: http://curl.haxx.se/bug/view.cgi?id=1323
HTTP POST: omit Content-Length if data size is unknown

This prevents sending a `Content-Length: -1` header, e.g this ocurred
with the following combination:

* standard HTTP POST (no chunked encoding),
* user-defined read function set,
* `CURLOPT_POSTFIELDSIZE(_LARGE)` NOT set.

With this fix it now behaves like HTTP PUT.
test1514: added - no more negative Content-Length (HTTP POST)

This covers changes from commit afd288b2.
contributors.sh: output list RELEASE-NOTES formatted
RELEASE-NOTES: synced with 12ecd56da77
SFTP: stat remote file also when CURLOPT_NOBODY is 1

Make it possible to call
curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &filesize)
and related functions on remote sftp:// files, without downloading them.

Reported-by: Yingwei Liu
Bug: http://curl.haxx.se/mail/lib-2014-01/0139.html
axtls: fix compiler warning on conversion ssize_t => int
RELEASE-NOTES: added missing bug ref
RELEASE-NOTES: added another missing bug ref
test1514: Got rid of a non-const initializer C99ism