Add support for symbol hiding disabled by default.

Add support for symbol hiding disabled by default.

Fully explained in libcrypto/README. TL;DR make sure libcrypto
and libssl's function calls internally and to each other are via
symbol names that won't get overridden by linking other libraries.

Mostly work by guenther@, which will currently be gated behind a
build setting NAMESPACE=yes. once we convert all the symbols to
this method we will do a major bump and pick up the changes.

ok tb@ jsing@