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