#22 customize wput resume offset

[fab128]

At this time wput always resumes broken transfers 512 bytes before the end of the file on the server. This is useful to avoir data corruption, but on the other hand it looks like many FTP servers on the market have a bug and does not take into account this resume offset (they always resume transfer from the end of the file, ending up to 512-bytes longer file). I have no problem with gftp which always resume from the end of the file on the server.
On way to easilsy solve this problem -- without having to harass my FAI so that they change their FTP server -- would be to add a command-line argument to customize this resume offset.