Make ex_data implementations internal

Make ex_data implementations internal

To state the obvious: library suffers from way too much extensibility. In
theory, applications can implement their own ex_data implementation. In
practice, none did. A glance at ex_data.c might give an idea as to why.
Make this internal so this particular turd can be replaced with something
slightly saner.

Also sync up the CRYPTO_EX_INDEX_* defines with OpenSSL - at least
the parts we support.

ok jsing