Add zeek::max_random() & fix misuse of RAND_MAX w/ zeek::random_number()

In deterministic mode, RAND_MAX is not related to the result of
zeek::random_number() (formerly bro_random()), but some logic was
using RAND_MAX as indication of the possible range of values.  The
new zeek::max_random() will give the correct upper-bound regardless
of whether deterministic-mode is used.

src/PacketFilter.h

@@ -34,7 +34,7 @@ public:
 private:
 	struct Filter {
 		uint32_t tcp_flags;
-		uint32_t probability;
+		double probability;
 	};
 
 	static void DeleteFilter(void* data);