Annotate places depending on strong random numbers

Annotate places depending on strong random numbers

This commit adds a new wrapper, random_get_bytes(), that is currently
defined to use os_get_random() as is. The places using
random_get_bytes() depend on the returned value being strong random
number, i.e., something that is infeasible for external device to
figure out. These values are used either directly as a key or as
nonces/challenges that are used as input for key derivation or
authentication.

The remaining direct uses of os_get_random() do not need as strong
random numbers to function correctly.