Change tarball making procedure

Since recently, OpenSSL tarballs are produced with 'make tar' rather
than 'make dist', as the latter has turned out to be more troublesome
than useful.

The next step to look at is why we would need to configure at all to
produce a Makefile just to produce a tarball.  After all, the tarball
should now only contain source files that are present even without
configuring.

Furthermore, the current method for producing tarballs is a bit
complex, and can be greatly simplified with the right tools.  Since we
have everything versioned with git, we might as well use the tool that
comes with it.

Added: util/mktar.sh, a simple script to produce OpenSSL tarballs.  It
takes the options --name to modify the prefix of the distribution, and
--tarfile tp modify the tarball file name specifically.

This also adds a few entries in .gitattributes to specify files that
should never end up in a distribution tarball.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7692)

(cherry picked from commit 8c209eee)