RAND_DRBG: add a function for setting the default DRBG type and flags

This commit adds a new api RAND_DRBG_set_defaults() which sets the
default type and flags for new DRBG instances. See also #5576.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5632)