PBKDF2 computation speedup (15-40%)

This commit contains some optimizations in PKCS5_PBKDF2_HMAC() and
HMAC_CTX_copy() functions which together makes PBKDF2 computations
faster by 15-40% according to my measurements made on x64 Linux with
both asm optimized and no-asm versions of SHA1, SHA256 and SHA512.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1708)