Add EVP_MD_CTX_legacy_clear()

Add EVP_MD_CTX_legacy_clear()

This is analogous to EVP_CIPHER_CTX_legacy_clear() and will serve as an
internal replacement for EVP_MD_CTX_init() until the conversion to heap
allocated ctx is completed. This way EVP_MD_CTX_init() can be changed to
match the OpenSSL 1.1 API.

ok jsing