Fix signature and semantics of EVP_{CIPHER,MD}_CTX_init()

Fix signature and semantics of EVP_{CIPHER,MD}_CTX_init()

When the EVP_CIPHER_CTX and the EVP_MD_CTX were still expected to live
on the stack, these initialization APIs were wrappers around memset.
In OpenSSL 1.1, somebody removed them and carelessly made _init() an
alias of _reset() aka _cleanup(). As a consequence, both signature and
semantics changed.

Unsurprisingly, there is now code out there that actually uses the new
semantics, which causes leaks on LibreSSL and older OpenSSL. This aligns
our _init() with OpenSSL 1.1 semantics.

ok jsing