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
I don't like returning an error/warning string from a low-level utility
function. That just moves the burden to every caller site. IMO the function
should be allowed to log if necessary.