Pool/Index data is not aligned

Correctly handle getaddrinfo return result

global: free the error message when exiting a thread

* Ensure alignment by copying the content into a structure on the stack

Remove unused buf variable from path/core test.

The getaddrinfo function indicates failure with a non-zero return code,
but this code is not necessarily negative. On platforms like Android
where the code is positive, a failed call causes libgit2 to segfault.

Factor 40 and 41 constants from source.

It looks like what Travis wants is a list of hashes, rather than a hash
with a list.

Fix `git_path_walk_up` to work with non-rooted paths

Replace void casts with GIT_UNUSED.

Provide a callback for certificate validation

travis: run valgrind only once

Instead of running valgrind on each job, half of which are in release
mode and don't have much usable information for valgrind, perform an
debug build as part of allowed_failures and run valgrind on that one,
which should speed up the feedback we get from the builds.

Skip it before we attempt to clone, as we would exit with -1 on systems
which do not have sshd running.

This brings us back in line with the other transports.

The user may have the data hashed as MD5 or SHA-1, so we should provide
both types for consumption.

Instead of using the libssh2 defines, provide our own, which eases usage
as we do not need to check whether libgit2 was built with libssh2 or not.

Instead of spreading the data in function arguments, some of which
aren't used for ssh and having a struct only for ssh, use a struct for
both, using a common parent to pass to the callback.

Test that the certificate check callback gets the right fingerprint from
the host we're connecting to.

This option make it easy to ignore anything about the server we're
connecting to, which is bad security practice. This was necessary as we
didn't use to expose detailed information about the certificate, but now
that we do, we should get rid of this.

If the user wants to ignore everything, they can still provide a
callback which ignores all the information passed.

This should make the mingw compiler happy.