Use os_get_random() for Shared Key authentication challenge

Use os_get_random() for Shared Key authentication challenge

Do not use the system clock or os_random() that uses a low quality PRNG
as part of the pseudo-random challenge in auth_shared_key(). The
construction can be improved upon by replacing it with a call to
os_get_random(), which uses a high quality PRNG.

Signed-off-by: Nick Lowe <nick.lowe@lugatech.com>