The current release process is not documented in any way. As a result,
it's not obvious how releases should be done at all, like e.g. which
locations need adjusting.

To fix this, let's introduce a new script that shall from now on be used
to do all releases. As input it gets the tree that shall be released,
the repository in which to do the release, credentials to
authenticate against GitHub and the new version. E.g. executing the
following will create a new release v0.32:

    $ ./script/release.py 0.32.0 --user pks-t --password ****

While the password may currently be your usual GitLab password, it's
recommended to use a personal access token intead.

The script will then perform the following steps:

    1. Verify that "include/git2/version.h" matches the new version.

    2. Verify that "docs/changelog.md" has a section for that new
       version.

    3. Extract the changelog entries for the current release from
       "docs/changelog.md".

    4. Generate two archives in "tar.gz" and "zip" format via "git
       archive" from the tree passed by the user. If no tree was passed,
       we will use "HEAD".

    5. Create the GitHub release using the extracted changelog entries
       as well as tag and name information derived from the version
       passed by the used.

    6. Upload both code archives to that release.

This should cover all steps required for a new release and thus ensures
that nothing is missing that shouldn't be.

valgrind will warn that OpenSSL will use undefined data in connect/read
when talking to certain other TLS stacks.  Thankfully, this only seems
to occur when gcc is the compiler, so hopefully valgrind is just
misunderstanding an optimization.  Regardless, suppress this warning.

libcrypto will read uninitialized memory as entropy.  Suppress warnings
from this behavior.

On Ubuntu, the combination of libgcrypt and libssh2 is quite old and
known to contain memory leaks. We thus have several functions listed in
our suppressions file that are known to leak. Due to a recent update of
libssh2 or libgcrypt, there now are new memory leaks caused by
libssh2_session_handshake and libssh2_init that cause the CI to fail.

Add a new suppression to fix the issue.

Right now, we have an awful hack in our test CI setup that extracts the
test command from CTest's output and then prepends the leak checker.
This is dependent on non-machine-parseable output from CMake and also
breaks on various ocassions, like for example when we have spaces in the
current path or when the path contains backslashes. Both conditions may
easily be triggered on Win32 systems, and in fact they do break our
Azure Pipelines builds.

Remove the awful hack in favour of a new CMake build option
"USE_LEAK_CHECKER". If specifying e.g. "-DUSE_LEAK_CHECKER=valgrind",
then we will set up all tests to be run under valgrind. Like this, we
can again simply execute ctest without needing to rely on evil sourcery.

Update GITERR and giterr usages in the static code analysis tools to use
the new names.