Add a best effort test to check shared library consistency

Our main development platforms are of the Unix family, which doesn't
have the same strictness regarding a shared library being consistent
with the contents of the ld script (.map file, on Linux and Solaris)
as Windows is with the contents of the .def file or VMS is with the
linker symb_vector option.

To eliminate surprises, we therefore need to make sure to check that
the contents of the .map file is matched with the shared library, at
least to check that the shared library isn't missing any symbols that
should be present.

This test isn't absolutely perfect, as it will only check the symbols
that would be present on Linux / Solaris and will therefore miss those
that would only appear on Windows or VMS.  On the other hand, those
platform specific are few and far apart in time, so it's not likely
they will pose a problem.

Reviewed-by: Matt Caswell <matt@openssl.org>