Install custom RAND_METHOD for fuzzing

Instead of setting a "magic" global variable to force RAND to keep
consistent state and always generate the same bytestream, have
the fuzzing code install its own RAND_METHOD that does this.  For
BN_RAND_DEBUG, we just don't do it; that debugging was about mucking
with BN's internal representation, not requiring predictable rand
bytes.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/4025)