Move random number re-seeding out of the hot path

Instead of checking if we need to re-seed for every isc_random call,
seed the random number generator in the libisc global initializer
and the per-thread initializer.

lib/isc/tests/random_test.c

@@ -87,6 +87,11 @@ _setup(void **state) {
 	result = isc_test_begin(NULL, true, 0);
 	assert_int_equal(result, ISC_R_SUCCESS);
 
+	/*
+	 * Ensure the RNG has been seeded.
+	 */
+	assert_int_not_equal(isc_random32(), 0);
+
 	return (0);
 }