Rewrite csrand_interval() as a wrapper around csrand_uniform()

The old code didn't produce very good random numbers.  It had a bias.
And that was from performing some unnecessary floating-point
calculations that overcomplicate the problem.

Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: Björn Esser <besser82@fedoraproject.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Joseph Myers <joseph@codesourcery.com>
Cc: Sam James <sam@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/prototypes.h

@@ -359,6 +359,7 @@ extern void pw_free (/*@out@*/ /*@only@*/struct passwd *pwent);
 /* csrand.c */
 unsigned long csrand (void);
 unsigned long csrand_uniform (unsigned long n);
+unsigned long csrand_interval (unsigned long min, unsigned long max);
 
 /* remove_tree.c */
 extern int remove_tree (const char *root, bool remove_root);