Add --no-check-transfers option

This option deactivates the checking of the MD5 sum of received/uploaded
files. This can be useful if you're downloading/uploading through a
proxy modifying the data on the fly, for example encrypting or
decrypting it.

Since MD5 sums are not checked anymore, there is a risk of getting
corrupted files.


Note: I updated the manpage manually, since running the format-manpage.pl script led to a number of related changes (apparently the manpage hasn't been updated for some time).