This means e.g. that a caller can say:

  RAND_SSLEay()->bytes(...)

and so on.  But in exchange for this convenience, I've changed the
signatures to be more BoringSSL-ish (|size_t| instead of |int|).
That's fine; |RAND_set_rand_method(SSLEay())| still works.  And by
works I mean "does nothing".

Change-Id: I35479b5efb759da910ce46e22298168b78c9edcf
Reviewed-on: https://boringssl-review.googlesource.com/5472
Reviewed-by: Adam Langley <agl@google.com>