Rewrite RSA_get_ex_new_index(3) and CRYPTO_set_ex_data(3) from scratch.

Rewrite RSA_get_ex_new_index(3) and CRYPTO_set_ex_data(3) from scratch.
The defects of the old pages were too numerous to list in full but included
vagueness, gaps, misleading statements, bad ordering, and duplication.
Use my Copyright since none of the text we inherited from OpenSSL remains.

Without doing a thorough review, tb@ thinks he likes the new pages
after quickly reading through both of them.