Add crypto_rand_int_range() and use it

Incidently, this fixes a bug where the maximum value was never used when
only using crypto_rand_int(). For instance this example below in
rendservice.c never gets to INTRO_POINT_LIFETIME_MAX_SECONDS.

  int intro_point_lifetime_seconds =
    INTRO_POINT_LIFETIME_MIN_SECONDS +
    crypto_rand_int(INTRO_POINT_LIFETIME_MAX_SECONDS -
                    INTRO_POINT_LIFETIME_MIN_SECONDS);

Signed-off-by: David Goulet <dgoulet@ev0ke.net>