random-seed: move pool size determination to random-util.[ch]

That way we can reuse it elsewhere.

src/basic/random-util.h

@@ -31,3 +31,9 @@ static inline uint32_t random_u32(void) {
 }
 
 int rdrand(unsigned long *ret);
+
+/* Some limits on the pool sizes when we deal with the kernel random pool */
+#define RANDOM_POOL_SIZE_MIN 512U
+#define RANDOM_POOL_SIZE_MAX (10U*1024U*1024U)
+
+size_t random_pool_size(void);