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/openssl.h

@@ -137,7 +137,7 @@
 
 #define EC_MAX_POINT_LEN ((528 * 2 / 8) + 1)
 
-#define _libssh2_random(buf, len) RAND_bytes ((buf), (len))
+#define _libssh2_random(buf, len) (RAND_bytes((buf), (len)) == 1 ? 0 : -1)
 
 #define libssh2_prepare_iovec(vec, len)  /* Empty. */