Switch from libusb to libusbx
screen: fixed the url from some FTP site in Germany to $GNU_URL.
mtr: updated sha1sum

Tarball changed sha1. After diffing the old and new one it seems only
*.o files and some configure files was removed that shouldn't have been
part of the original tarball.
Merge pull request #217 from Ratler/libusbx

Switch from libusb to libusbx
Merge pull request #219 from dagbrown/screen_url

screen: fixed the url from some FTP site in Germany to $GNU_URL.
Merge remote-tracking branch 'upstream/master'