ex_data part 2: doc fixes and CRYPTO_free_ex_index.

Add CRYPTO_free_ex_index (for shared libraries)
Unify and complete the documentation for all "ex_data" API's and objects.
Replace xxx_get_ex_new_index functions with a macro.
Added an exdata test.
Renamed the ex_data internal datatypes.

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