DRBG: delay initialization of DRBG method until instantiation

DRBG: delay initialization of DRBG method until instantiation

Previously, the initialization was done immediately in RAND_DRBG_set(),
which is also called in RAND_DRBG_uninstantiate().

This made it difficult for the FIPS DRBG self test to verify that the
internal state had been zeroized, because it had the side effect that
the drbg->data structure was reinitialized immediately.

To solve the problem, RAND_DRBG_set() has been split in two parts

    static int rand_drbg_set(RAND_DRBG *drbg, int type, unsigned int flags);
    static int rand_drbg_init_method(RAND_DRBG *drbg);

and only the first part is called from RAND_DRBG_uninstantiate().

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11111)