add GetRandBytes() as wrapper for RAND_bytes()

- add a small wrapper in util around RAND_bytes() and replace with
  GetRandBytes() in the code to log errors from calling RAND_bytes()
- remove OpenSSL header rand.h where no longer needed
- move rand functions from util to new random.h/.cpp
BTW; this function can be moved to the implementation file and made static. It's only used in random seeding.
I think this means that the header doesn't need to include compat.h and time.h anymore.