Make wget capable of starting downloads from a specified position.

This patch adds an option `--start-pos' for specifying starting position
of a HTTP or FTP download.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Tests: fix TYPE and RETR command handling.

 - FTPServer.pm's handling of TYPE command would ignore binary mode
   transfer request.
 - The FTP server would run into dead loop sending the same content
   forever.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Tests: exclude existing files from the check of unexpected downloads.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Tests: Add test cases for option --start-pos.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
Tests: Add constraint on https for --https-only test.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
NEWS: cite --start-pos