OpenSSL exception as agreed, with new SPDX expression

OpenSSL exception as agreed, with new SPDX expression

Licence text approved by co-contributor Mark Wooding in personal
email.

DPDX expression roughly following
  https://spdx.github.io/spdx-spec/v2.3/using-SPDX-short-identifiers-in-source-files/
which doesn't contemplate using LicenseRef for an exception, but I
think the meaning is clear, and it is useful to do it this way.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>