Cleanup: rename EVP_MD_CTX_(create|init|destroy) to EVP_MD_CTX_(new|reset|free)

Looking over names, it seems like we usually use names ending with
_new and _free as object constructors and destructors.  Also, since
EVP_MD_CTX_init is now used to reset a EVP_MD_CTX, it might as well be
named accordingly.

Reviewed-by: Rich Salz <rsalz@openssl.org>