Add GRND_INSECURE from Linux 5.6 to sys/random.h

This patch adds the GRND_INSECURE constant from Linux 5.6 to glibc's
sys/random.h.  This is also added to the documentation.  The constant
acts as a no-op for the Hurd implementation (as that doesn't check
whether the flags are known), which is semantically fine, while older
Linux kernels reject unknown flags with an EINVAL error.

Tested for x86_64.

manual/crypt.texi

@@ -301,6 +301,9 @@ booted and the randomness source has not yet been initialized.
 @item GRND_NONBLOCK
 Instead of blocking, return to the caller immediately if no data is
 available.
+
+@item GRND_INSECURE
+Write random data that may not be cryptographically secure.
 @end table
 
 Unlike @code{getentropy}, the @code{getrandom} function is a

stdlib/sys/random.h

@@ -25,6 +25,7 @@
 /* Flags for use with getrandom.  */
 #define GRND_NONBLOCK 0x01
 #define GRND_RANDOM 0x02
+#define GRND_INSECURE 0x04
 
 __BEGIN_DECLS