RSA_NULL used to be a compile option allowing the RSA interfaces to be
compiled-in, with nonfunctional code, to be able to cope with the RSA
patent.

However, we don't use this option, and the RSA patent has expired more than 10
years ago, so just drop this piece.
remove unused variables getentropy for OS X

ok beck@
Update to match the current state of crypto/rand, and remove MLINKS for
RAND_event and RAND_screen.