crypto/rand: rename drbg_rand.c to drbg_ctr.c

The generic part of the FIPS DRBG was implemented in fips_drbg_lib.c and the
algorithm specific parts in fips_drbg_<alg>.c for <alg> in {ctr, hash, hmac}.
Additionally, there was the module fips_drbg_rand.c which contained 'gluing'
code between the RAND_METHOD api and the FIPS DRBG.

When the FIPS code was ported to master in #4019, for some reason the ctr-drbg
implementation from fips_drbg_ctr.c ended up in drbg_rand.c instead of drbg_ctr.c.

This commit renames the module drbg_rand.c back to drbg_ctr.c, thereby restoring
a simple relationship between the original fips modules and the drbg modules
in master:

 fips_drbg_lib.c    =>  drbg_lib.c    /* generic part of implementation */
 fips_drbg_<alg>.c  =>  drbg_<alg>.c  /* algorithm specific implementations */

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