Automate updating of vendored libraries

[enkore]

This is currently a manual process;

• libssh: copy new source into tree, update tarballs and signatures, update Dockerfiles
• krb5, openssl, etc.: update tarballs, update all Dockerfiles

A script that does these things automatically and verifies signatures automatically would be nice, even nicer would be some kind of CI automation.

See also https://github.com/ParallelSSH/ssh-python/issues/86