#17 Check CRC of yenc-parts

[TH]

It would be nice if nget would check the CRC of yenc
encoded articles, and try to get the article from a
different server if the CRC is bad. While I realise that you
don't want to mess with the decoding, this could be done
separately while downloading the article, and there is a
ready to use code example in http://www.yenc.
org/ydecode-c.txt