Rewrite crypto/ex_data

Removed ability to set ex_data impl at runtime.  This removed these
three functions:
    const CRYPTO_EX_DATA_IMPL *CRYPTO_get_ex_data_implementation(void);
    int CRYPTO_set_ex_data_implementation(const CRYPTO_EX_DATA_IMPL *i);
    int CRYPTO_ex_data_new_class(void);
It is no longer possible to change the ex_data implementation at
runtime.  (Luckily those functions were never documented :)

Also removed the ability to add new exdata "classes."  We don't believe
this received much (if any) use, since you can't add it to OpenSSL objects,
and there are probably better (native) methods for developers to add
their own extensible data, if they really need that.

Replaced the internal hash table (of per-"class" stacks) with a simple
indexed array.  Reserved an index for "app" application.

Each API used to take the lock twice; now it only locks once.

Use local stack storage for function pointers, rather than malloc,
if possible (i.e., number of ex_data items is under a dozen).

Make CRYPTO_EX_DATA_FUNCS opaque/internal.

Also fixes RT3710; index zero is reserved.

Reviewed-by: Richard Levitte <levitte@openssl.org>