Making use of sha256sum

[Olav Vitters <olav bkor dhs org>]

As md5sum is insecure, I've switched install-module to make use of
sha256sum instead. There were some changes on the server to have it use
sha1sum, I rather use sha256sum.

jhbuild has been updated to support sha256 (or whatever) as well. I plan
to generate .sha256sum files on 'ftp.gnome.org' as well (instead of
md5sum files). I don't want any trace of md5 to remain, this to ensure
people don't rely on an outdated hash mechanism.

Note: As it is release time, I'm going to avoid any further changes
(releng scripts) until after the release.

PS: install-module lives in the releng git module (should probably add a
comment in that file).

-- 
Regards,
Olav