Fix _libssh2_random() silently discarding errors (#520)

Notes:
* Make _libssh2_random return code consistent

Previously, _libssh2_random was advertized in HACKING.CRYPTO as
returning `void` (and was implemented that way in os400qc3.c), but that
was in other crypto backends a lie; _libssh2_random is (a macro
expanding) to an int-value expression or function.

Moreover, that returned code was:
  — 0 or success, -1 on error for the MbedTLS & WinCNG crypto backends
But also:
  — 1 on success, -1 or 0 on error for the OpenSSL backend!
  – 1 on success, error cannot happen for libgcrypt!

This commit makes explicit that _libssh2_random can fail (because most of
the underlying crypto functions can indeed fail!), and it makes its result
code consistent: 0 on success, -1 on error.

This is related to issue #519 https://github.com/libssh2/libssh2/issues/519
It fixes the first half of it.

* Don't silent errors of _libssh2_random

Make sure to check the returned code of _libssh2_random(), and
propagates any failure.

A new LIBSSH_ERROR_RANDGEN constant is added to libssh2.h
None of the existing error constants seemed fit.

This commit is related to d74285b684
and to https://github.com/libssh2/libssh2/issues/519 (see the issue
for more info.)  It closes #519.

Credit:
Paul Capron

src/kex.c

@@ -3176,7 +3176,11 @@ static int kexinit(LIBSSH2_SESSION * session)
 
         *(s++) = SSH_MSG_KEXINIT;
 
-        _libssh2_random(s, 16);
+        if(_libssh2_random(s, 16)) {
+            return _libssh2_error(session, LIBSSH2_ERROR_RANDGEN,
+                                  "Unable to get random bytes "
+                                  "for KEXINIT cookie");
+        }
         s += 16;
 
         /* Ennumerating through these lists twice is probably (certainly?)