random-util: add random_u64_range() that acquires a random number from a certain range, unbiased

So far we have been quite sloppy with this and ignored modulus and range
bias. Let's do something about, and add the option to do better.

src/basic/random-util.h

@@ -40,3 +40,5 @@ int rdrand(unsigned long *ret);
 size_t random_pool_size(void);
 
 int random_write_entropy(int fd, const void *seed, size_t size, bool credit);
+
+int random_u64_range(uint64_t max);