Make libssh2 work again on os400. (#118)

* os400: minimum supported OS version is now V6R1.
Do not log compiler informational messages.

* Implement crypto backend specific Diffie-Hellman computation.

This feature is now needed on os400 because the QC3 library does not
implement bn_mod_exp() natively. Up to now, this function was emulated using
an RSA encryption, but commits ca5222ea81 and
7934c9ce2a (CVE-2016-0787) broke the emulation
because QC3 only supports RSA exponents up to 512 bits.

Happily, QC3 supports a native API for Diffie-Hellman computation, with
opaque random value: this commit implements the use of this API and, as a
side effect, enables support of this feature for any other crypto backend that
would use it.

A "generic" Diffie-Hellman computation internal API supports crypto backends
not implementing their own: this generic API uses the same functions as before.

* Fix typos in docs/HACKING.CRYPTO.

src/libssh2_priv.h

@@ -248,7 +248,7 @@ typedef struct kmdhgGPshakex_state_t
     size_t s_packet_len;
     size_t tmp_len;
     _libssh2_bn_ctx *ctx;
-    _libssh2_bn *x;
+    _libssh2_dh_ctx x;
     _libssh2_bn *e;
     _libssh2_bn *f;
     _libssh2_bn *k;