Add a new ssh client implementation based on Apach MINA sshd 2.0.0.

This implementation uses JGit's own config file parser and host entry
resolver. Code inspection of the Apache MINA implementation revealed
a few bugs or idiosyncrasies that immediately would re-introduce bugs
already fixed in the past in JGit.

Apache MINA sshd is not without quirks either, and I had to configure
and override more than I had expected. But at least it was all doable
in clean ways.

Apache MINA boasts support for Bouncy Castle, so in theory this should
open the way to using more ssh key algorithms, such as ed25519.

The implementation is in a separate bundle and is still not used in
the core org.eclipse.jgit bundle. The tests re-use the ssh tests from
the core test bundle.

Bug: 520927
Change-Id: Ib35e73c35799140fe050d1ff4fb18d0d3596580e
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

Add support for git-receive-pack to the ssh git server and add two
new tests for pushing.

This actually uncovered an undocumented requirement in TransportSftp:
the FTP rename operation assumes POSIX semantics, i.e., that the
target is removed. This works as written only for servers that
support and advertise the "posix-rename@openssh.com" FTP extension.

Our little Apache MINA server does not advertise this extension.

Fix the FtpChannel implementation for Jsch to handle this case in a
meaningful way so that it can pass the new "push over sftp" test.

Add more tests to test the behavior of server host key checking.

Also refactor the tests generally to separate better the test
framework from the actual tests.

Bug: 520927
Change-Id: Ia4bb85e17ddacde7b36ee8c2d5d454bbfa66dfc3
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

Change-Id: Id5ba87b24e8a29157d49e9e5b151d65bdbcab37a
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

Add a simple ssh git server based on Apache MINA sshd, and use it
in new tests that verify ssh operations and in particular a number
of bugs that had cropped up over time in JSch.

The git server supports fetching only, and sftp access.

The tests are all in an abstract base class; the concrete JschSshTest
class only provides ssh-specific test setup. So the same tests could
be run easily also with some other ssh client.

Bug: 520927
Change-Id: Ide6687b717fb497a29fc83f22b07390a26dfce1d
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>