This introduces a test suite for libssh2. It runs OpenSSH in a Docker
container because that works well on Windows (via docker-machine) as
well as Linux. Presumably it works on Mac too with docker-machine, but
I've not tested that.
Because the test suite is docker-machine aware, you can also run it
against a cloud provider, for more realistic network testing, by setting
your cloud provider as your active docker machine. The Appveyor CI setup
in this commit does that because Appveyor doesn't support docker
locally.
Do not create symbolic links off the build directory. Recent autotools
verify that out-of-source build works even if the source directory tree
is not writable.
