Rearrange RNG code slightly

In preparation of exporting getentropy/getrandom to userspace, rearrange
code a bit:

- Define RtlGenRandom in ntdll.h.

- Drop calls to getentropy in favor of RtlGenRandom (fhandler_socket,
  fhandler_dev_random).

- Add try/except blocks in fhandler_dev_random to return EFAULT rather
  than crashing if buffer pointer is invalid.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/ntdll.h

@@ -1608,4 +1608,10 @@ extern "C"
 
   }
 }
+
+/* There's a bug in ntsecapi.h (Mingw as well as MSFT).  SystemFunction036
+   is, in fact, a WINAPI function, but it's not defined as such.  Therefore
+   we have to do it correctly here.  *Strictly*, this is not ntdll... */
+#define RtlGenRandom SystemFunction036
+extern "C" BOOLEAN WINAPI RtlGenRandom (PVOID, ULONG);
 #endif