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
That would be consistent, but no need to do it in this pull, let's leave code movements and functional changes separate.