Wire up HMAC to raw private key methods

Wire up HMAC to raw private key methods

Obviously, the brilliant API design kitchen decided that an interface
carrying public and private key in its name (so that every sane person
thinks of asymmetric cryptography), is also perfectly suitable for MACs.
Wire up HMAC since Ruby's OpenSSL gem uses these bindings if the build
system detects that EVP_PKEY_new_raw_public_key() is available in evp.h.

While there, also add the missing pub_cmp() ameth, which obviously
treats two things as equal by returning 1.

Reported by jeremy and anton, fixes regress/lib/libssl/openssl-ruby tests

ok jsing