Add random serial# support.

Add -rand_serial to CA command and "serial_rand" config option.

Up RAND_BITS to 159, and comment why: now confirms to CABForum
guidelines (Ballot 164) as well as IETF RFC 5280 (PKIX).

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4185)