[DEV] random: use RtlGenRandom() on Windows

rand_s() is readily available in Windows's CRT,
accessible through <stdlib.h>. Unfortunately its
output is limited to an integer, thus requiring
multiple calls to fill a large buffer.

RtlGenRandom() is the other hand can accomodate
any buffer size, but is a pure Windows (NT) function,
not available in Windows's CRT, as it's declared
in <ntsecapi.h> and available in advapi32.dll.

And according to Microsoft documentation, the
availability of the function is unclear, it's
future too.

https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom

But according to Rust community  the function is
used in Firefox, Chromium, etc, thus it's unlikely
to be removed.

rust-random/rand#111

Especially since the rand_s() documentation state
its uses RtlGenRandom():

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/rand-s?view=vs-2019

So rand_s() uses RtlGenRandom(), hence make use of
advapi32.dll, then there's no need to use rand_s()
when RtlGenRandom() is available.

This patch makes use of RtlGenRandom(), and fallback
to rand_s() if the former fail.

Note: for newer Windows versions (>= 10), RtlGenRandom()
could be replaced by newer, recommanded functions:

   BCryptGenRandom(BCRYPT_RNG_ALG_HANDLE,
                   buffer, len,
                   0);

https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
https://docs.microsoft.com/en-us/windows/win32/seccng/cng-algorithm-pseudo-handles

Author: Yves-Marie Morgan

src/random.c

@@ -31,6 +31,10 @@
 
 #ifdef _WIN32
 #  define _CRT_RAND_S
+#  define WIN32_LEAN_AND_MEAN
+#  include <windows.h>
+#  include <ntsecapi.h>
+#  define HAVE_RTLGENRANDOM 1
 #endif  /* _WIN32 */
 
 #include "futils/random.h"
@@ -50,6 +54,14 @@ int futils_random_bytes(void *buffer, size_t len)
 	uint8_t *p = buffer;
 	int ret = 0;
 	unsigned int val = 0;
+
+#ifdef HAVE_RTLGENRANDOM
+	if (!RtlGenRandom(buffer, len))
+		ULOGE("RtlGenRandom");
+	else
+		return 0;
+#endif
+
 	while (len) {
 		size_t chunk = sizeof(val);
 		if (chunk > len)