util: import sha1 implementation from OpenBSD

util: import sha1 implementation from OpenBSD

At the moment we support 5+ different implementations each with varying
amount of bugs - from thread safely problems [1], to outright broken
implementation(s) [2]

In order to accommodate these we have 150+ lines of configure script and
extra two configure toggles. Whist an actual implementation being
~200loc and our current compat wrapping ~250.

Let's not forget that different people use different code paths, thus
effectively makes it harder to test and debug since the default
implementation is automatically detected.

To minimise all these lovely experiences, import the "100% Public
Domain" OpenBSD sha1 implementation. Clearly document any changes needed
to get building correctly, since many/most of those can be upstreamed
making future syncs easier.

As an added bonus this will avoid all the 'fun' experiences trying to
integrate it with the Android and SCons builds.

v2: Manually expand __BEGIN_DECLS/__END_DECLS and document (Tapani).

Furthermore it seems that some games (or surrounding runtime) static
link against OpenSSL resulting in conflicts. For more information see
the discussion thread [3]

Bugzilla [1]: https://bugs.freedesktop.org/show_bug.cgi?id=94904
Bugzilla [2]: https://bugs.freedesktop.org/show_bug.cgi?id=97967
[3] https://lists.freedesktop.org/archives/mesa-dev/2017-January/140748.html
Cc: Mark Janes <mark.a.janes@intel.com>
Cc: Vinson Lee <vlee@freedesktop.org>
Cc: Tapani Pälli <tapani.palli@intel.com>
Cc: Jonathan Gray <jsg@jsg.id.au>
Tested-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Tapani Pälli <tapani.palli@intel.com> (v1)
Acked-by: Jason Ekstrand <jason@jlekstrand.net> (v1)