crypto/rand: restore the generic DRBG implementation

The DRGB concept described in NIST SP 800-90A provides for having different
algorithms to generate random output. In fact, the FIPS object module used to
implement three of them, CTR DRBG, HASH DRBG and HMAC DRBG.

When the FIPS code was ported to master in #4019, two of the three algorithms
were dropped, and together with those the entire code that made RAND_DRBG
generic was removed, since only one concrete implementation was left.

This commit restores the original generic implementation of the DRBG, making it
possible again to add additional implementations using different algorithms
(like RAND_DRBG_CHACHA20) in the future.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4998)